“Science has established that it is virtually certain that increases of atmospheric CO2 due to burning of fossil fuels will cause climate change that will have substantial adverse impacts on humanity and on natural systems. Therefore, immediate, stringent measures to suppress the burning of fossil fuels are both justified and necessary.” — David Karoly
In December, 2015, over 190 countries met at the 21st Conference of Parties to the United Nations Framework Convention on Climate Change (UNFCCC) in Paris. They agreed on individual “Nationally Determined Contributions” to reduce global greenhouse gas emissions in order to “prevent dangerous anthropogenic interference with the climate system.”
Each country had its own reasons for supporting this agreement. All are aware of the many independent, peer-reviewed, comprehensive scientific assessments of the science of climate change, vulnerabilities and adaptation to its impacts, and approaches to mitigate human-caused climate change. These assessments include the series of five reports on “America’s Climate Choices,” prepared by the US National Research Council in 2010 and 2011; a joint report, “Climate change: evidence and causes,” from the Royal Society of London and the US National Academy of Sciences in 2014; as well as an extended series of assessments by the Intergovernmental Panel on Climate Change (IPCC) since 1990, culminating in the six volumes of its “Fifth Assessment Report,” released in 2013 and 2014.
Of course, not all scientists agree with all the conclusions of these assessment reports. Some may agree with some but not other conclusions. Some may disagree because they have not assessed the vast number of relevant peer-reviewed publications or because they don’t have relevant expertise to make such assessments. Some may focus on selected pieces of evidence that they perceive rebut the conclusions, without undertaking a comprehensive assessment. Others may emphasize the uncertainties in the science of climate change associated with the complexities and natural variability of the climate system.
Nevertheless, an overwhelming consensus of climate scientists agree that human-caused climate change is happening and that global warming will continue throughout the current century, with many adverse impacts on human and natural systems. This consensus is based on a vast body of scientific evidence and many thousands of peer-reviewed scientific publications that have been assessed in the above reports. It is the consensus of the evidence and the peer-reviewed publications that is important, which leads to the consensus of climate scientists.
While scientific interest in global warming and climate change has grown rapidly over the last 40 years, it is not a new science. In the nineteenth century, scientists were aware of the large extent of ice sheets over northern Europe and northern North America during the last ice age and sought to explain the necessary colder temperatures needed for the growth of those ice sheets. Studies showed the important role of trace gases in the atmosphere that absorbed and re-emitted long wavelength or infrared radiation and therefore affected the surface temperature of the Earth. This would later become know as the “greenhouse effect.” A Swedish scientist, Svante Arrhenius, argued in 1896 that it was carbon dioxide variations, not the more common water vapor in the atmosphere, that were key to determining the variations in global surface temperature. Arrhenius also suggested that continued burning of coal through industrial activity would lead to increasing carbon dioxide in the atmosphere and to global warming. He estimated that a doubling of atmospheric carbon dioxide concentrations would lead to global warming of 5–6° C, but was not concerned because of the very long time that it would take to achieve such a doubling given the rates of coal burning at the time.
Of course, there have been many scientific updates to the estimates by Arrhenius, not the least because emissions of greenhouse gases from burning coal and other fossil fuels grew very rapidly through the twentieth century.
In the following sections, I provide brief summaries of the evidence supporting the six key points below, which underpin my Opening Statement above.
- Observed global warming of the climate system over the last hundred years is beyond any reasonable doubt.
- Observed increases in greenhouse gases in the atmosphere are primarily due to human activity; burning fossil fuels, other industrial activity, deforestation and land clearing.
- The observed large-scale increase in surface temperature across the globe since the mid-20th century is primarily due to human activity, the increase of greenhouse gases in the atmosphere and other human impacts on the climate system.
- There will continue to be significant global warming over the 21st century, with its magnitude depending on the emissions of greenhouse gases from human activity.
- There are substantial adverse impacts on human and natural systems from global warming.
- Rapid, substantial and sustained reductions in greenhouse gas emissions from human activities are needed to slow global warming and stabilize global temperature at a level that would minimize dangerous human influence on the climate system.
1. Observed global warming is beyond reasonable doubt
“Global warming” means an increase in the temperature or the heat content of the global climate system. The most common indicator of global warming is the average of the surface or near-surface temperature across the globe, including the sea surface temperature and the near-surface air temperature over land. However, this is not the only indicator of warming of the global climate system. When heat is added to the climate system for any reason, most of that heat (about 90%) goes into warming the oceans, some goes into melting ice sheets and glaciers, some goes into warming the land, and a small fraction warms the atmosphere.
When we consider climate change or global warming, it is important to consider changes over periods of 30 years or longer. The World Meteorological Organization (WMO) considers a climate normal to be the average conditions of temperature, rainfall and other weather variables over a period of 30 years. The most recent climate normal is for the period 1981–2010. We consider such an extended period to reduce the effects of natural variability from year to year and decade to decade, much of which is random and associated with exchanges of heat between the ocean and the atmosphere.
The WMO recently released the latest estimates of the global average surface temperature for 2015 from three centers —NASA and NOAA in the US and the UK Meteorological Office —as shown below, and compared them with annual variations over the period since 1850.
Twenty Fifteen is the hottest year in all three datasets and 2014 is the second-hottest year. Part of this record heat in 2015 is due the occurrence of a strong El Ni ño in 2015, which was associated with hotter-than-normal sea surface temperatures in the equatorial Pacific Ocean and the release of heat from the ocean into the atmosphere. El Ni ño is part of the natural variability of the climate system. The previous El Ni ño was in 1997–98 and was even stronger than the 2015–16 El Ni ño, but the global average temperature then was about 0.2° C lower than in 2015.
To reduce the influence of this natural variability, we should consider longer-term averages. The most recent decade, 2006–2015, is the warmest decade in the observational record since 1850. The most recent 30-year period, 1986–2015, is also the hottest such period since 1850.
The long-term temperature change between the nineteenth century and the early twenty-first century can be estimated from this graph above, and shows global warming of about 0.9° C. This is much larger than the year-to-year variations of temperature, the natural variability of up to 0.3° C from year to year. Hence, the global warming signal is much larger than the noise of chaotic natural climate variability.
There have been criticisms of the quality of the long-term record of temperature from weather observing stations on land. Some of these criticisms include the possible warming influence due to the growth of cities and urban areas around weather stations that had originally been in rural locations. Others include the influence of adjustments made to the original observations to seek to produce a high-quality homogeneous data record by removing the influences of site moves and instrument changes. These concerns have been addressed by the scientists who have processed the data and have been shown to have minimal impact on the long-term global warming trend.
In the graph above, the differences of the independent estimates of the global average temperature anomalies from the three centers are smaller than 0.1° C, arising from different analysis methods. The uncertainties in the estimates associated with uncertainties in the measurements at each location and the global data coverage, shown in the gray shading, are about 0.1° C in the recent period, but much larger, up to 0.2° C, in the late nineteenth century, when there was less-complete global data coverage.
There are many other indicators of warming of the climate system, apart from increases in global mean temperature, or average temperature, on land. These include long-term increases in both sea surface temperatures and air temperatures observed from ships since the mid-nineteenth century. The average warming of sea surface temperatures is less than the average warming over land over the last 100 years, as expected from the greater mixing of heat into deeper waters in the ocean and greater evaporation from the ocean surface.
Long-term observations of the length of glaciers show marked reductions from the late nineteenth century to the present for all glaciers that have changed in length, as expected in a warming climate. Some glaciers fed by the ice sheets in Greenland and Antarctica have not changed in length.
Long-term observations of sea level from tide gauges at many coastal and island sites show a significant increase in sea level of about 20 cm from the late nineteenth century to the present. This sea level rise is an indicator of warming of the ocean waters and melting of ice on land, both due to global warming.
Other indicators of global warming have shorter observational records and are only available since the mid-twentieth century or the onset of satellite observations. These include reductions in snow cover over land in the Northern Hemisphere, particularly in spring, increases in the average heat content in the deeper ocean, increases in global average air temperature throughout the lower atmosphere, and increases in the water vapor content of the lower atmosphere, all of which are expected due to global warming.
While there is no doubt about the warming of the global climate system over the last hundred years, this global warming does not mean that every location has warmed at the same rate or even that no locations have cooled over this period. There are large spatial variations in the magnitude of the warming trend over the last hundred years but almost all places on the Earth show statistically significant warming trends over the last hundred years, as shown in Figure 3 below. However, a few locations where long-term observations are available do not show significant warming and one or two show cooling that is not significant.
Two regions of interest that do not show significant warming since 1901 are the North Atlantic Ocean, which shows a slight cooling trend, and the southeast United States, which shows negligible warming.
Spatial variations in the long-term surface temperature trends depend on a number of different factors, including the characteristics of the underlying surface, natural variability of the atmospheric and oceanic circulation, and regional temperatures and regional changes in the climate forcing factors such as urbanization, land clearing, irrigation, or air pollution. The cause of the warming hole in the North Atlantic Ocean is likely due to increased mixing of deep and cooler water to the surface. However, the causes of the warming hole in the southeast United States are not clear and appear to be a combination of changes in the winds in response to sea surface temperature changes and changes in land surface characteristics.
2. Increases in greenhouse gases are due to human activity
In addition to warming of the climate system, the other significant change to the climate system over the last 100 years has been the dramatic increase in the concentrations of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and other long-lived greenhouse gases in the atmosphere.
The background concentrations of these gases in the air, away from local sources, have been directly monitored at multiple sites around the globe only for about the last 40 to 50 years. However, longer-term measurements can be obtained from air bubbles trapped in snow and ice in the ice sheets in Greenland and Antarctica. These show relatively stable concentrations of these greenhouse gases over the last 10,000 years, followed by a rapid increase since around the mid-nineteenth century.
As Figure 4, below, shows, the global average concentration of carbon dioxide in 2014 was 397 parts per million (ppm), about 44% higher than pre-industrial levels. The current global average concentration of methane is 151% higher and for nitrous oxide is 21% higher than pre-industrial levels.
To understand the causes of the increase in carbon dioxide and other long-lived greenhouse gases in the atmosphere, we can look at multiple lines of evidence.
The observed concentration of carbon dioxide in the atmosphere is about 400 parts per million. Over the last 800,000 years, during ice ages and warmer interglacial periods, the CO2 concentration was never above 300 parts per million, as shown by the air extracted from bubbles trapped in ice from Greenland and Antarctica. This shows that the recent large increases in CO2 can’t be explained by natural variability. However, a few scientists suggest other causes, including that the increase is due to CO2 from underwater volcanoes (Ian Plimer) or is due to the release of CO2 from the oceans as they have warmed over the past century (Murry Salby), which is the reason for the increase of atmospheric CO2 from an ice age to an interglacial period.
The increase in atmospheric CO2 is associated with a decline in the isotopic ratio of carbon-13 to carbon-12 in the CO2, as expected for carbon that has been processed through plants by photosynthesis. This is exactly what would result from the burning of fossil fuels and from land clearing. In addition, there has been an observed small decrease in the oxygen concentration in the atmosphere, exactly as expected from burning fossil fuels. Such a change in isotopic ratios in the atmospheric CO2 and the decline in atmospheric oxygen cannot be explained by carbon dioxide coming from underwater volcanoes or from the oceans.
The increase in atmospheric carbon dioxide over the last 40 years agrees very well with the increase expected from emissions associated with burning fossil fuels, land clearing, and industrial activity, less the additional uptake of carbon dioxide into the oceans and the land ecosystems due to the higher concentrations.
Hence, human activity since the industrial revolution, associated with burning fossil fuels, industrial activity, agriculture, and land clearing, is the cause of the increases in carbon dioxide and other long-lived greenhouse gases in the atmosphere over the last 100 years.
3. Most of the observed global warming since the mid-twentieth century is due to human activity
To determine the likely causes of the observed increase in global average temperatures over the last century, or the last 50 years, we have to assess the relative magnitudes of the contributions from natural chaotic variability of the climate system, from increasing greenhouse gases, from changing sunlight from the sun, and other possible factors.
It is important to recognize again that the natural variability of surface temperatures is much greater at regional and local scales than at the global scale. Hence, if we wish to identify the possible causes of global warming, it is important to consider variability of global or hemispheric average temperatures, and not regional temperature variability.
First, we assess whether the observed warming trend of about 0.9° C over the last 100 years is unusual, or whether it could be explained by natural variability alone. To do this, we must use estimates of global and hemispheric average temperature variations over the last millennium from palaeoclimate studies. These all show that the observed warming trend over the twentieth century averaged over the Northern Hemisphere is much greater than in any other century over the last 1,000 years. Fewer studies have undertaken similar analysis of Southern Hemisphere average temperature variations over the last millennium, but these also show that the warming over the twentieth century is unusual. Hence, the global-scale warming over the last 100 years very likely cannot be explained by natural variability alone.
Next, climate model simulations have been used to assess the relative importance of different forcing factors on the climate system and how well they explain the observed global warming. The simulations are driven by natural forcing factors, such as changes in solar radiation and volcanic aerosols, as well as human-caused changes in greenhouse gases and human activity-related climate forcing factors, including industrial aerosols and land use change.
Multiple simulations from more than 30 different global climate models have been run as part of an internationally coordinated climate modeling experiment, the World Climate Research Programme’s “Coupled Model Intercomparison Project Phase 5” (CMIP5). This included multiple studies evaluating the performance of the climate models, as well as simulations of the climate of the twentieth century and simulations of the future climate to 2100 and later for prescribed future greenhouse gas concentration scenarios.
Simulations of the historical climate variations from 1860 to 2010 with these climate models are shown below. The simulations are started in 1860 and allowed to run freely, with the only external input being changes in boundary conditions that represent the natural (solar radiation and volcanic aerosols) and anthropogenic (greenhouse gases, aerosols, and land use change) forcing factors. The results from three different sets of simulations are shown below, first for model simulations with combined natural and anthropogenic climate forcings, then with natural forcings only, and finally with forcing by increasing greenhouse gases alone. The thick black lines show three observational estimates, while the thick colored lines show the average of all the different climate model simulations and the thin colored lines show individual model simulations.
The observed significant cooling for one to two years after major volcanic eruptions —Santa Maria (1903), Agung (1963), El Chicon (1982), and Pinatubo (1991) —is simulated very well. The observed global mean temperature variations throughout the whole period lie within the range of all the model simulations with combined forcings, indicating the models simulate well the chaotic interannual variability of global mean temperature. There is very good agreement between the observed long-term global warming since the late nineteenth century and the average global warming across all the model simulations for combined natural and anthropogenic forcing.
Global mean temperature variations from observations and global climate model simulations, 1860–2010
The role of natural climate forcing in the observed global warming can be assessed from the simulations with natural climate forcings alone, as shown in the middle panel. While these show the intermittent short-term cooling associated with major volcanic eruptions, they do not show the observed long-term global warming, particularly since the mid-twentieth century. The observed global warming cannot be explained by natural climate variability or natural climate forcings, such as changes in solar irradiance, as simulated by these climate models.
For the model simulations forced by greenhouse gases increases alone (shown in the bottom panel), the simulated global warming is larger than observed. This leads to the conclusion that other anthropogenic forcing factors, particularly aerosols associated with industrial activity, have a cooling impact on the global climate.
Of course, a small number of scientists say that the climate models are tuned to simulate the recent observed warming, but are unreliable for projecting future warming trends. Others say that they show too much global warming, because the observed warming from 1998 to 2010 was very small, while the simulated warming continued, if you consider the average across all the climate model simulations. As shown already when considering the observed global mean temperature variations, there is large natural variability in global mean temperature in the observations and the models. The observed departure in 2010 from the multi-model mean is no larger than in 1910 or in 1940, and is well within the envelope of all the model simulations.
In addition to the simple consideration of the observed and simulated global mean temperature variations, more detailed studies have been undertaken to assess the quantitative contributions of different forcing factors to the observed large-scale warming of the climate system. These are sometimes called “fingerprint detection and attribution studies,” because they use the large-scale spatial patterns (fingerprints) of the temperature changes expected due to different forcing factors. They then estimate the changes in magnitude of the fingerprints in the observed large-scale temperature change and thus determine the magnitudes of contributions from the different forcings. This does not use the model-estimated magnitudes, just the spatial patterns of the temperature responses to the different forcings.
Figure 8, below, shows the contributions to the observed trend in global mean temperature over the period 1951–2010 from the different forcing factors, together with the observed trend. The increase in greenhouse gases from human activity is the major cause of the observed warming trend. The likely contributions from internal variability and from natural factors, such as changes in volcanoes or changes in sunlight from the sun, are much smaller and close to zero.
The best agreement with the observed warming trend is for the combined anthropogenic forcings, with warming due to greenhouse gases alone being greater than observed, offset somewhat by cooling due to other anthropogenic forcings, such as aerosols. It is important to note that the space-time fingerprint of the response to aerosol forcings can be distinguished from that for greenhouse gases alone and their magnitudes estimated separately.
We can also confidently exclude changes in solar activity as a contributor to recent observed climate change using the fingerprint of the response to increases of solar radiation in the observed pattern of changes in temperature in the stratosphere and in the lower atmosphere. Increases in solar radiation would lead to warming in the stratosphere due to absorption by oxygen and ozone in the upper atmosphere and warming in the lower atmosphere and at the surface. However, the observed pattern of temperature change over the last 50 years shows cooling in the lower stratosphere and warming in the lower atmosphere and at the surface, exactly as expected from human impacts on the climate.
We don’t need to use climate model simulations to show that changes in solar radiation cannot be a major contributor to the observed warming trends by using simple physically based arguments. The contribution of solar radiation to the surface energy balance is greater in summer than in winter and in daytime than at night, so increases in solar radiation would be expected to lead to more warming in summer than in winter and in daytime than at night on land. However, the observed surface warming trend over land areas over the period since the mid-twentieth century shows more warming in winter than in summer and more warming at night than in the daytime, opposite to what would be expected from increases in solar radiation.
4. Global warming will continue over the twenty-first century
In order to assess the range of possible future climate changes to 2100 and beyond, we cannot use a time machine or a crystal ball. Global climate models provide the best tool available for doing this, because they were shown in the previous section to be able to simulate the observed large-scale climate change well over the last 100 years or more.
Global climate models have been used to simulate a range of possible future climate scenarios to 2100 and even longer for some models. These simulations start from the historical simulations up to 2005 and then use a series of plausible greenhouse gas concentration and industrial aerosol scenarios to 2100, called “Representative Concentration Pathways” (RCP).
Each of the RCPs is labeled with the change in net radiative forcing (in W/m2) in 2100 associated with the greenhouse gas concentrations and aerosols, from the lowest (RCP2.6), a stringent emission reduction scenario, to the highest (RCP8.5), a high greenhouse gas emission scenario. The highest scenario, RCP8.5, represents business-as-usual emissions from burning fossil fuels, industrial activity, and agriculture, with carbon dioxide concentrations reaching more than 930 ppm by 2100 and continuing to rise rapidly. The other scenarios all represent plausible emission reduction scenarios, with increasing levels of emission mitigation policy, leading to stabilization of greenhouse gas concentrations in the atmosphere at different levels at some time between 2060 and 2200. The lowest emission and concentration scenario is RCP2.6, which has carbon dioxide concentrations peaking at about 440 ppm in 2050 and then falling to about 420 ppm by 2100. For all these RCP scenarios, industrial aerosols and particulate emissions fall rapidly from present day levels to near zero by 2100.
Multiple simulations from more than 30 different global climate models have been run as part of international CMIP5 climate modeling project with these different RCP scenarios. These simulations provide projections of climate change to 2100 for many different climate variables, including changes in surface temperature, rainfall, winds, sea ice, and sea level, as well as many others. The simulations represent the natural internal variability of the climate system, as well as the response to changes in external climate forcings, as shown in the previous section. Hence, it is important to consider averages over an extended period, such as over 20 years, and over multiple simulations, to provide the best estimate of future climate change.
Uncertainties in projected future climate change arise for three separate sources: uncertainties in the future human-related and natural climate forcing factors; natural internal climate variability at annual and decadal timescales; and differences between the climate responses to a given forcing between different climate models. The CMIP5 simulations allow us to assess each of these uncertainties, by considering the differences between simulations for different RCPs, differences between simulations from the same model for different atmospheric and oceanic initial conditions to assess the role of chaotic internal variability, and differences between groups of simulations from different models.
The projected changes in the global mean surface temperature and sea level over the twenty-first century are shown below relative to 1986–2005. For the high-emission scenario (RCP8.5), the best estimate of additional global warming in 2080–2100 is 3.7° C, with a likely range of 2.6° to 4.8° C. Given that the observed global warming from the second half of the nineteenth century to 1986–2005 was about 0.6° C, this corresponds to best estimate global warming in the late twenty-first century of 4.3° C above pre-industrial levels, with a 17% likelihood of global warming greater than 5.4° C. This magnitude of global warming would make the global temperature higher than at any time over the last 50,000,000 years.
For the rapid emission reduction scenario (RCP2.6), global warming peaks around 2050 and global surface temperature remains stable for the rest of the twenty-first century. The best estimate of additional global warming in 2080–2100 is 1.0° C, with a likely range of 0.3° C to 1.7° C above 1986–2005 levels. Relative to pre-industrial temperatures, this corresponds to a best estimate of global warming of about 1.6° C, with a 17% likelihood of global warming greater than 2.3° C. Hence, even under this rapid emission reduction scenario, there is a substantial risk that global warming would exceed the internationally agreed target of limiting global warming to below 2 C above pre-industrial levels.
It is important to recognize that natural year-to-year and decadal variability of surface temperature will continue, even as global warming increases. This natural variability is larger at regional scales and at shorter time scales.
In addition, there will be substantial regional differences in the global pattern of surface temperature change, as shown below. The largest warming is expected at higher latitudes in both hemispheres and over inland continental areas, with less warming over the North Atlantic Ocean and Southern Ocean, where additional heat is mixed into the deep ocean. The average warming over land is about 25% higher than the global average warming, and about 50% higher than the warming over land. This means that, averaged over land areas, the best estimate of warming under the rapid emission reduction scenario would be 2.0° C and under the high-emission scenario would be 5.4° C.
As global warming continues through the twenty-first century, there will be many associated changes to the climate system. Global-mean sea level rise will continue and likely accelerate, as shown above. Global-mean sea level has already risen by more than 0.2m by the end of the twentieth century relative to 1850–1900. For the high-emission scenario (RCP8.5), the best estimate of sea level rise by late this century is 0.63 m relative to 1986–2005, with a likely range of 0.45 m to 0.82 m. This corresponds to best estimate of global sea level rise of 0.83 m relative to pre-industrial levels, with more than 17% chance of sea level rise greater than 1.0 m by the end of the twenty-first century. For the rapid emission reduction scenario (RCP2.6), global sea level does not stabilize like global warming, but continues rising to end the end of the twenty-first century and beyond. Under this low-emission scenario, the best estimate of sea level rise late in the twenty-first century is 0.60 m relative to pre-industrial levels, with a 17% chance of sea level rise greater than 0.75 m.
The continued global sea level rise beyond the end of this century due to global warming occurs because of the slow melting of the ice sheets on Greenland and Antarctica and the slow penetration of heat into the deep ocean. Both of these processes will continue for centuries even after global mean temperature increases have stopped. The current ice sheet on Greenland holds the equivalent of about 7 m of global sea level rise and is likely to melt completely over an extended period for global warming higher than 4° C.
There are many other high-confidence changes that are expected in response to increases in global warming. There will be more frequent and more intense heat waves and hot extremes over most land areas, as well as fewer cold extremes, although some cold extremes in winter may still occur. It is very likely that sea ice extent in the Arctic will continue to decline during the twenty-first century, together with decreases in springtime snow cover. Heavy one-day rain and snow events will very likely become more frequent and more intense over most land areas in middle latitudes and in wet tropical regions.
All components of the climate system are likely to be affected by global warming in some way, but it may not be possible to distinguish some of these changes from natural variability at short time scales or in some regions.
5. Many adverse impacts result from global warming
There are many reasons to be concerned about the adverse impacts of human-caused climate change on society and on natural ecosystems. While there may be some short-term benefits of higher CO2 in some sectors and a slightly warmer climate, all comprehensive assessments of the impacts of climate change across many sectors show many reasons for concern.
Our society, our agricultural systems, our cities, and our natural ecosystems have all developed in a period with a relatively stable climate over the last 2,000 years. However, the rate of global warming is so rapid that our society and the systems on which we depend are becoming stressed. As noted in the previous section, even greater changes are expected over the rest of this century.
The recent US National Climate Assessment in 2014 has highlighted the impacts of climate change across the United States, including impacts already observed and those that will be much greater in the coming decades as global warming continues.
Many impacts of global warming and widespread climate change have already been observed in human and natural systems. Increases in global and regional temperatures have led to major changes in the regions in which some specific plants grow, affecting agriculture and natural ecosystems. As the climate warms, the growing zones for plants move upward in elevation and poleward to try to maintain the same climate. This will continue and is likely to lead to the loss of some natural ecosystems, such as coral reefs and alpine ecosystems, and dramatic changes in agricultural regions, particularly in already hot areas like the tropics.
Global warming has led to increases in hot extremes and heatwaves, affecting human health and leading to crop and animal losses, as well as increases in the occurrence and intensity of wild fires in some regions.
Increases in global temperature have led to global sea level rise, flooding coastal areas and causing coastal erosion and pollution of coastal freshwater systems with seawater. The impacts of storm surges, combined with global and regional sea level rise, were clearly demonstrated by the storm surge impacts of Hurricane Sandy on New York City and the east coast of the United States. Expected sea level rise by the end of this century for even the smallest projected global warming will lead to the annual flooding of many hundreds of millions of people and the complete loss of some low-lying island countries.
For human health impacts from global warming, the US National Climate Assessment in 2014 concludes:
Climate change threatens human health and well-being in many ways, including impacts from increased extreme weather events, wildfire, decreased air quality, threats to mental health, and illnesses transmitted by food, water, and disease-carriers such as mosquitoes and ticks. Some of these health impacts are already underway in the United States….
Certain people and communities are especially vulnerable, including children, the elderly, the sick, the poor, and some communities of color.
Indeed, many studies have concluded that the impacts of climate change are not distributed uniformly. The impacts are greater for the poor, disadvantaged, and indigenous populations around the world, partly because they have greater exposure and are more vulnerable to the impacts of climate change and partly because they have fewer resources to adapt to or to avoid these impacts.
There are regional differences in the impacts of climate change, associated with the region-specific climate changes, but there are many similarities in the impacts of global warming across different regions. The graphic below summarizes some of the major impacts of climate change in the state of Victoria, in southeast Australia. It shows that many of the impacts of global warming and sea level rise are common around the world, more hot extremes, increases in fire weather, declines in snow cover, and increases in coastal flooding due to sea level rise.
There are some possible benefits from higher carbon dioxide in the atmosphere and a slightly warmer atmosphere for some sectors and some regions. However, all comprehensive assessments have concluded that these benefits are likely to only occur for moderate levels of global warming. For example, in colder climates, an increase in temperature is likely to increase the growing season if there are no other associated adverse impacts, such as changes in water resources or increases in insect pests or diseases. For larger warming magnitudes, the changes are likely to be sufficient to lead to major disruptive shifts in ecosystems or agricultural zones.
The increase in carbon dioxide concentrations in the atmosphere has some potential benefits for plants because carbon dioxide is essential for photosynthesis. Plants grown in an atmosphere with higher carbon dioxide have faster growth rates and lower water use, assuming there are no other limits on growth. However, often plant growth is limited by other factors, including lack of nutrients or lack of water or temperature extremes, including heat waves, so that increases in carbon dioxide don’t provide unlimited benefits to plant growth. In addition, higher carbon dioxide levels have been shown to lead to lower levels of protein production even when plant growth increases.
One of the other major impacts of climate change due to increasing carbon dioxide concentrations is the increase in carbon dioxide dissolved in the oceans. As shown below, the dissolved carbon dioxide in the upper waters of the ocean has increased in parallel with the increase in atmospheric concentration. As the oceans absorb more carbon dioxide, they become less basic (or more acidic), with a higher concentration of carbonic acid. This can be seen in the decrease in pH of ocean water by about 0.1 units over the last 30 years. While 0.1 units sounds small, it corresponds to about a 30% increase in the hydrogen ion availability in ocean water.
There are major impacts on marine organisms due to the increase in acidity, particularly on shell-forming marine life. More acidic ocean water reduces the ability of marine life to extract calcium carbonate from sea water and form exoskeletons or shells. This has major adverse impacts on shellfish and corals and these impacts have already been observed in some regions. In addition to the adverse impacts on coral reefs from global warming and oceanic heat waves leading to coral bleaching, ocean acidification is another major stressor on coral reefs.
While the magnitude of the ocean acidification observed so far is a reduction of about 0.1 in pH, the projected changes are much larger and depend on future concentrations of carbon dioxide in the atmosphere. For the high-emission scenario (RCP8.5), the projected decline in pH is greater than 0.4 units, corresponding to a more than doubling of hydrogen ion availability. The last time atmospheric concentrations were that high, geological evidence indicates that there was a mass extinction of benthic marine life.
As the magnitude of global warming increases, so do the impacts of climate change and the risks of abrupt and irreversible regional changes in natural systems. While we may be able to adapt and minimize some of the adverse impacts, there are limits to the effectiveness of adaptation, especially with greater magnitudes and rates of global warming.
Countries around the world have agreed on action to limit climate change because of their concerns about the adverse impacts of global warming and the associated “dangerous anthropogenic interference with the climate system.” They are aware of some potential benefits of warming but are more concerned about the risks of adverse impacts.
6. Substantial reductions in greenhouse gas emissions are needed to minimize dangerous global warming
As noted in the Topic 2 above, human activity is the cause of the increases in carbon dioxide and other long-lived greenhouse gases in the atmosphere over the last 100 years. Specific human activities producing these greenhouse gases include burning fossil fuels to generate electricity or for transport, industrial activities like making cement, agriculture, and land clearing.
Limiting global warming to any level requires stabilizing greenhouse gas concentrations in the atmosphere. This is the objective of the United Nations Framework Convention on Climate Change agreed by all countries and coming into force in 1992: “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”
Stabilizing greenhouse gas concentrations in the atmosphere to minimize dangerous global warming requires rapid, substantial, and sustained reductions in greenhouse gas emissions from human activity. The net emissions (sources minus sinks) of greenhouse gases into the atmosphere from human activity need to fall from present levels to near zero as quickly as possible.
In Topic 4 above, on projections of future global warming, the rapid emission reduction scenario (RCP2.6) showed future climate change with about a 30% risk of global warming exceeding 2° C above pre-industrial levels. The greenhouse gas emissions for that scenario, as well as for the continued high-emission scenario (RCP8.5), are shown below.
The rapid emission reduction scenario (RCP2.6) shows roughly constant annual net emissions of long-lived greenhouse gases from 2015 to 2020 of about 45 billion tons CO2-eq per year, falling to less than 20 billion tons CO2-eq per year by 2050 and to less than 10 billion tons CO2-eq per year by 2080. It is important to recognize that these include projected methane and nitrous oxide emissions from agriculture, which may be much harder to reduce than carbon dioxide emissions from fossil fuel burning. Indeed, different versions of the RCP2.6 emission scenario generally show negative net carbon dioxide emissions before the end of the twenty-first century, with zero net emissions from burning fossil fuels and various forms of carbon dioxide capture and storage to remove carbon dioxide from the atmosphere.
In contrast, the continued high-emission scenario (RCP8.5) shows greenhouse gas emissions more than doubling from present days levels to more than 100 billion tons CO2-eq per year by 2080.
Cumulative emissions of carbon dioxide and other long-lived greenhouse gases from human activity largely determine the magnitude of global warming over the twenty-first century, as shown in Figure 15, below. Using this, scientists have estimated the global budgets of carbon dioxide emissions consistent with global warming not exceeding 2° C, with a range of probabilities. There is a finite amount of carbon dioxide emissions from human activities that is consistent with avoiding global warming of 2° C or more and avoiding dangerous climate change. For higher confidence of avoiding warming of two degrees, the carbon budget is smaller.
Limiting global warming due to human-related emissions of carbon dioxide to less than 2° C with a probability of more than 66% requires cumulative CO2 emissions of less than about 1000 GtC (3670 GtCO2) since 1870, as shown below. This amount is reduced to about 790 GtC (2900 GtCO2) when non-CO2 greenhouse gas emissions consistent with the RCP2.6 scenario are taken into account. In addition, about 515 GtC (1890 GtCO2) have already been emitted from 1870 to 2011. Hence, total global carbon dioxide emissions after 2011 have to be limited to less than about 275 GtC (1010 GtCO2) if global warming is likely to be limited to less than two degrees. At current rates of emissions, this total carbon budget would be used up before 2040.
An implication of the cumulative carbon budget determining global warming is that any delay in reducing carbon dioxide emissions uses up more of the budget and will cause more warming. In other words, a decision not to reduce carbon dioxide emissions is a decision to allow greater global warming.
As burning fossil fuels is the greatest source of carbon dioxide emissions from human activity, it is critical to phase out the burning of fossil fuels as quickly as possibly or introduce technologies that capture all the carbon dioxide when fossil fuels are burnt and prevent all of it from entering the atmosphere.
At the Paris Conference of the UN Framework Convention on Climate Change (UNFCCC) in December, 2015, 190 countries agreed to limit their future emissions of greenhouse gases with the objective of limiting global warming to less than two degrees above pre-industrial levels and minimizing dangerous climate change.
The key points presented above are just a small fraction of the vast body of evidence that support the scientific conclusions on global warming accepted by all the scientific Academies and by all the governments around the world.
Science has established that it is virtually certain that increases of atmospheric CO2 due to burning of fossil fuels will cause climate change that will have substantial adverse impacts on humanity and on natural systems. Therefore, immediate stringent measures to suppress the burning of fossil fuels are both justified and necessary.
6. Svante Arrhenius, “On the Influence of Carbonic Acid in the Air Upon the Temperature of the Ground,” Philosophical Magazine, 1896, 41: 237–276.
8. D.L. Hartmann, et al., “2013: Observations: Atmosphere and Surface,” in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. by T.F. Stocker, et al. Cambridge & New York: Cambridge University Press, 2014; pp.159–254. For graph, see IPCC Report Graphics.
9. “IPCC (2013) Summary for Policymakers,” in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. by T.F. Stocker, et al. Cambridge & New York: Cambridge University Press, 2014.
12. N.L. Bindoff, P.A. Stott, et al., “Detection and Attribution of Climate Change: from Global to Regional” [PDF], in Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, ed. by T.F. Stocker, et al. Cambridge & New York: Cambridge University Press, 2014.
13. “IPCC (2014) Summary for Policymakers,” in Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. Geneva, Switzerland: IPCC, 2014. For graph, see IPCC Report Graphics.
15. Jerry M. Melillo, Terese (T.C.) Richmond, and Gary W. Yohe, eds., 2014: Highlights of Climate Change Impacts in the United States: The Third National Climate Assessment. (US Global Change Research Program, 2014).
16. Will Steffen and Lesley Hughes, “The Critical Decade 2013: Climate change science, risks and responses,” [JPG] (Australia’s Climate Commission, 2013).
17. “Highlights of Climate Change Impacts in the United States: The Third National Climate Assessment” (GlobalChange.gov, 2014). For graph, see here [JPG].