Why are climate experts so sure that human activity is dangerously warming the planet?
By William Collins, Robert Coleman, James Haywood, Martin R. Manning, and Philip Mott
Some of the authors of the latest and most comprehensive international report, which reviewed the scientific evidence for climate change, summarize in this article the arguments on the subject and discuss the points of uncertainty that still remain open.
In the life of a scientist studying climate change, "eureka" moments are very rare. Instead, progress is achieved through the painstaking compilation of evidence obtained from temperature measurements, satellite observations, or climate model experiments. The data is checked again and again and the ideas are tested again and again. Do the observations match the predicted changes? Is an alternative explanation possible? Good climate scientists, like all good scientists, strive to ensure a high level of proof for any discovery.
And the evidence that the climate is changing is indeed increasing as the climate record lengthens, as our understanding of the climate system improves, and as the climate models become more reliable. Over the past 20 years, evidence of the influence of humans on the climate has continuously accumulated, and together with this, the certainty in the scientific community has increased that the climate change occurring recently is a real change, and it is likely that a much greater change will occur in the future. This growing certainty is well reflected in the latest report of the Intergovernmental Panel on Climate Change (IPCC), which is the fourth in a series of recent assessments on the subject. The report was written and reviewed by hundreds of scientists around the world.
In February 2007, the panel published a condensed version of the first part of the report, which analyzes the scientific basis of climate change. The "summary for policymakers", as the short version is called, is intended not only for statesmen but also for ordinary people, and it conveys this message in an unambiguous way: scientists are more convinced than ever that humans have influenced the climate and that another human-caused change is imminent. Although the report states that some of the expected changes are inevitable, the analysis of the data also shows that we still have to fix the future, especially the distant future. The degree of predicted future change depends on what humans choose to do regarding greenhouse gas emissions.
The assessment, which is based on the physical sciences, focuses on four issues: the factors driving climate change, the observed changes in the climate system, understanding the cause-and-effect relationships, and implications for future changes. Since the IPCC's previous assessment in 2001, research has improved considerably in all these areas. In the following pages, we will present the main findings documenting the scope of the change, and leading to the inevitable conclusion that human activity is the cause of it.
Factors driving climate change
Human activities have increased the concentration of many gases in the atmosphere, mainly carbon dioxide, methane, nitrogen oxides and halogenated carbon compounds (gases that in the past were widely used in refrigerators and in the production of sprays). Such gases trap thermal energy (heat) in the atmosphere, and this, well known as the "greenhouse effect", leads to global warming. Atmospheric concentrations of carbon dioxide, methane and nitrogen oxides were more or less constant for 10,000 years until their sudden and accelerated increase in the last 200 years [see illustrations in the right part of the box on page 67]. The rate of increase in carbon dioxide concentration in the last 10 years was higher than in any other decade since continuous monitoring of the atmosphere began in the 50s. The concentrations of this gas are today about 20% higher than their level in the pre-industrial period (which can be determined by analyzing air bubbles trapped in ice cores). Methane levels are approximately 35 times higher than pre-industrial levels, and nitrogen oxide levels are approximately 2.5% higher.
How can we be sure that humans are responsible for these increases in concentrations? Some greenhouse gases (eg, most halogenated carbon compounds) have no natural origin. And as for the other gases, two important observations demonstrate the influence of humanity. First, geographical differences in gas concentrations reveal that their sources are mainly concentrated in the continents of the northern and more populated half of the planet. Second, an isotopic analysis, which makes it possible to differentiate between the emission sources, shows that most of the addition of carbon dioxide to the atmosphere comes from the burning of fossil fuels (coal, oil and natural gas). Methane and nitrogen oxides result from agricultural activity and the burning of mineral fuel.
Climate scientists use the term "radiative forcing" to quantitatively determine the effect of the increase in the concentration of gases on the climate. Radiative forcing is the change imposed on the Earth's energy balance from pre-industrial times to the present day. (The physical units of radiative forcing are watts per square meter.) Positive forcing causes warming and negative forcing causes cooling. We are able to determine quite precisely the rate of radiative forcing involved in the presence of greenhouse gases over time because we know their concentration in the atmosphere, their distribution in space and their physical interaction with radiation.
Climate change is not caused only by an increase in the concentration of greenhouse gases. There are other mechanisms, natural and man-operated, that participate in the change process. The natural causes include changes in solar activity and large volcanic eruptions. The report identifies several additional forcing mechanisms that man activates, such as the release of microscopic particles called aerosols, ozone in the stratosphere and troposphere, the albedo (the rate of reflection of light) of the surface, and aircraft trails, although the effects of these forcing mechanisms are less certain than those of greenhouse gases [see left figure in the box on the opposite page].
The climatic implications of the relationship between anthropogenic aerosols and cloud albedo is the issue about which researchers are least certain. Aerosols react with clouds in complicated ways and cause them to become brighter, that is, to reflect more sunlight into space. Another source of uncertainty comes from the direct effect of aerosols of human origin: how much do the particles directly reflect light and how much do they absorb light? The overall effects of the aerosols may result in a cooling that somewhat balances the warming effect of the long-lived greenhouse gases. But to what extent? Can their influence cancel the warming? Since the IPCC report in 2001 there has been great scientific progress in many fields, and with it the ability to determine the degree of uncertainty involved in each of the forcing mechanisms. This is done through a combination of simulations and observations from many studies. Today we can therefore confidently estimate the extent of the total human impact. According to the best estimates, the degree of human influence is ten times greater than the natural radiation forcing caused by the changes in the activity of the sun.
We are therefore more certain than before that the net radiative forcing acting on the climate system is a positive constraint. This increasing certainty also coincides with the observations that indicate the warming of the earth, and later in the article we will discuss these observations. The constraints can be likened to a tug-of-war: the positive constraints pull the Earth towards a warmer climate, and the negative towards a cooler climate. The result is clear: we know the strength of the contestants better than ever. They are pulling the Earth towards a warmer climate and will continue to do so with increasing force as the strongest puller in the rope, the greenhouse effect, gains strength.
Observed climatic changes
The abundance of improved and updated observations available to the authors of the IPCC report in 2007 enabled a more comprehensive assessment of the changes compared to the previous reports. The record collected by observations shows that 11 of the past 12 years have been the warmest years since reliable tracking began around 1850. The probability of a random sequence of such warm years is extremely small. Changes in three important variables - global temperature, sea level and snow cover in the northern hemisphere [see box on page 68] - point to warming, although they differ from each other in the small details. The previous IPCC assessment reported a warming trend of 0.6±0.2 degrees Celsius during the period between 1901 and 2000. Due to the recent considerable warming, the updated estimate for the hundred years between 1906 and 2005 rose to 0.74±0.18 degrees Celsius. It should be noted that only in the years 2005-1956 did the temperature rise by 0.65±0.15 degrees Celsius, a figure that emphasizes the fact that most of the warming in the 20th century took place in the last 50 years. The climate, of course, continues to fluctuate above and below these rising averages, and the extreme events also change according to the average trend: frosty days and cold days and nights have become rarer, while heat waves and warm days and nights have become more common.
The properties of the climate system are not measured solely through the known data of average temperature, amount of precipitation, etc., but also through the state of the sea and the cryosphere (the sea ice, the large ice sheets covering Greenland and Antarctica, glaciers, snow, frozen ground and the ice in lakes and rivers.) Complex interactions between the different parts of the climate system are a fundamental element of climate change. For example, the reduction of sea ice increases the absorption of heat in the oceans and the flow of heat between the oceans and the atmosphere, changes that may also affect cloudiness and the amount of precipitation.
In general, many additional observations are consistent with the observed warming, and reflect heat transfer from the atmosphere to the other components of the climate system. Spring snow cover in the mid-northern latitudes, which naturally diminishes as temperatures rise toward summer, shrank surprisingly in about 1988 and has remained small ever since. The phenomenon is worrisome because the snow cover is important for maintaining soil moisture and feeding water sources in many areas.
In the oceans, we clearly see warming trends, which, as expected, become more and more blurred with depth. These changes show that the oceans absorbed more than 80% of the heat added to the climate system. This warming is the main contributor to the rise of the sea level: the sea level rises because the volume of water increases as its temperature increases, and because the melting water of the glaciers and ice sheets is added to it. Satellite observations that began in 1993 allow a more accurate calculation of sea level rise. Today it is estimated that the water level rose at a rate of 3.1±0.7 millimeters per year between 1993 and 2003. Similar increases have also been recorded in the past several decades, so longer satellite monitoring will be needed to determine for sure if the rate of sea level rise has accelerated. In the Arctic region, a considerable reduction in the extent of sea ice has been measured since 1978 (2.7%±0.6% per decade on an annual average, 7.4%±2.4% per decade on average during the summer periods). In the last decades, other phenomena have also been observed, such as an increase in the temperature of the frozen lands all year round and a decrease in the extent of the glaciers in the world and in the area with little ice in Greenland and Antarctica. Unfortunately, until the last few decades, proper monitoring of many of these variables was not carried out, therefore the dates of the beginning of their documentation are different.
The hydrological changes also generally correspond to the warming. Water vapor is the most powerful greenhouse gas, but unlike the other greenhouse gases, its concentration is determined primarily by temperature. In general, since the 80s, and perhaps even earlier, the concentration of water vapor has increased. The amount of precipitation varies greatly from region to region, but several large regions of the world have seen an increase, including the eastern regions of North and South America, northern Europe, and central and northern Asia. Conversely, drying was observed in the south of the Sahara, in the Mediterranean region, in South Africa and in parts of South Asia. The salinity of the sea can be used as a giant rain gauge. In the middle and high latitudes there was a decrease in the salinity of the upper layers of the oceans, while in the low latitudes the water became saltier. This is consistent with changes in large-scale precipitation patterns.
It is possible to gain additional important insights into the way the climate system works with and without human influence by reconstructing the climate in the distant past, called paleoclimate. This is done by measuring the thickness of the rings in tree trunks and other means. Such an analysis shows that the high temperatures of the last 50 years are unusual, at least for the last 1,300 years. The hottest period in the years 1950-700 was between 950 and 1100, and even then the temperature was several tenths of a degree lower than the prevailing average since 1980.
Analysis of the observed changes
Although we are largely certain that human activity causes positive radiative forcing and that the climate is indeed changing, can we definitively link the two? It is a question of attributing cause to effect: is human activity the main factor responsible for the observed climatic changes, or perhaps they are the result of another factor, such as some natural constraint, or simply spontaneous variability of the climate system? A 2001 IPCC report stated that most of the warming that has occurred since the mid-20th century is "probably" (more than 66% likely) caused by humans. The 2007 report takes a big step forward and raises the probability level to "almost certainly" (more than 90%).
The growing confidence comes from a plethora of unrelated innovations. First, the observational record has been extended by about 5 years, and the global temperature has increased during this period in good agreement with the predictions of all previous IPCC reports, which were based on warming due to greenhouse gases. In addition, changes in other aspects of the climate, such as atmospheric circulation or deep sea temperatures, were taken into account. These changes paint a broader picture that fits the claim that humans are interfering with the climate. The computerized climate models, which are the key to cause-and-effect attribution studies, have also improved and reached a level where they are able to present very faithfully the current climate and that of the recent past. And finally, some important discrepancies discovered in the observations used in the previous report have been largely resolved.
The most serious discrepancy was apparently between the measurement of the ground temperature (which showed considerable warming in recent decades, a figure consistent with human influence) and the atmospheric measurements made by balloons and satellites (which showed only a little of the expected warming). Several new studies that examined the satellite and balloon data have resolved the contradiction to a large extent - and the data now point to warming both on the surface and in the atmosphere.
The perfect way to examine the cause of climate change is an experiment on another real world, where they recreate the 20th century but keep constant (and not rising) concentrations of greenhouse gases. But since such an experiment is obviously not possible, the scientists therefore do the closest thing to it: they simulate the past using a computerized climate model.
On this topic, since the last IPCC report, considerable progress has been made in two important areas, progress that has increased the reliability of the use of models to determine the cause of climate changes and to predict the future. The first is the development of a comprehensive and well-coordinated collection of simulations collected from 18 research groups around the world, with which it is possible to reconstruct the evolution of the climate on Earth in the past and to predict its evolution in the future. Using many models helps to determine in each simulation the amount of influence of uncertainties on different climatic processes. Although some of the processes are well understood and properly represented in physical equations (for example, the circulation of the atmosphere and oceans or the scattering of sunlight and heat), some of the crucial components of the climate system are poorly understood, such as clouds, marine eddies, and evaporation from vegetation. The modelers evaluate these components using simpler representations called parameterizations. The basic reason for developing a multi-model collection for the IPCC assessment is the need to understand how much this uncertainty affects the attribution of cause to effect and the prediction of climate change. The collection of models used for the latest evaluation includes an unprecedented number of models and experiments performed using them.
The second development is their integration into models of more realistic representations of climatic processes. Among these processes: the behavior of atmospheric aerosols, the dynamics (movement) of sea ice and the exchange of water and energy between the land and the atmosphere. Additional models now also include the important types of aerosols and the interactions between the aerosols and the clouds.
When scientists use models to assess the cause of a climatic phenomenon, they first run simulations that evaluate what would have happened if only "natural" factors, such as changes in solar radiation or volcanic eruptions, had been at work in the last 100 years. So they run the simulations that also include human impacts, such as greenhouse gases and aerosols. The results of these experiments are amazing [see box below]. The models that include only natural constraints are not able to explain the global warming observed since the middle of the 20th century, but they do explain it when they also include the constraints of human origin. Broad-scale patterns of temperature change are also consistent with models that include all forcings.
Two patterns serve as a fingerprint for human influence. In the first one, we see that the warming over the land is greater than over the oceans, and that the surface of the sea has warmed more than the deep layers. This temperature pattern corresponds to warming due to greenhouse gases in the atmosphere: the sea warms more slowly due to its high heat capacity. The warming also shows that a large amount of heat has been absorbed by the sea, a phenomenon that demonstrates the fact that the energy balance of the planet has been disturbed. The second pattern of change shows that while the troposphere (the lower atmosphere layer) has warmed, the stratosphere above it has cooled. If changes in the sun were the dominant forcing on warming, we would expect warming of both layers of the atmosphere. On the other hand, the actual observation contradicts this scenario but corresponds exactly to what is expected from a combination of an increase in the concentration of greenhouse gases and a decrease in the concentration of ozone in the stratosphere. The accumulated evidence, after careful statistical analysis, provides a solid basis for us to be confident that human influence is behind the observed global warming. Alternative proposals regarding cosmic rays that could affect the clouds, as well as the climate, were based on correlations with a limited database, and did not stand up to the test when additional data were added to them. Besides, their physical mechanisms remain speculative.
And what about smaller scale changes? When the standards in time and space are reduced, the assessment of causes becomes more difficult. The problem arises from the fact that natural temperature changes on a small scale are less able to be "smoothed by averaging" and therefore more easily tend to mask the change. Still, the continued warming causes the signs of change to begin to emerge even on smaller scales. The report's findings show that human activity has had a significant effect on the temperature on a continental scale, and not just globally, on all continents except Antarctica.
Human influence is also evident in extreme events, such as extremely hot or cold nights and heat waves. This does not imply, of course, that a certain extreme event (such as the heat wave that hit Europe in 2003) was simply "caused" by human-induced climate change. Such events are complex, and their causes are multiple. But it can be said that human activity has increased, not decreased, the chance that such events will occur.
Predicting future changes
How is the climate expected to change during the 21st century? The answer to this crucial question is determined by simulations of climate models based on predictions of future emissions of greenhouse gases and aerosols. The simulations imply that if greenhouse gas emissions continue at the current rate or exceed it, the climate changes in the future will most likely be greater than the changes already observed during the 20th century. Even if we were to immediately cut emissions so that greenhouse gas concentrations remain as they are today, the climate will continue to change for hundreds of years. This climatic persistence results from a combination of several factors, including the heat capacity of the oceans and the thousands of years required until the natural cycles manage the heat and carbon dioxide in the depths of the ocean and bring about an equilibrium in the new conditions.
And in more detail, the models predict that over the next 20 years, if emissions remain within a reasonable and conceivable range, the global temperature will rise at an average rate of 0.2 degrees Celsius per decade, a rate similar to that observed in the last 30 years. About half of this warming, in the short term, stems from the "swing" in which the climatic system reacts to the current atmospheric concentrations of greenhouse gases.
However, in the long term, the prediction of warming in the 21st century is greatly influenced by the future rate of emissions. The projections span many scenarios that include moderate to rapid economic growth and lower, or higher, dependence on fossil fuels. The best estimates of the global temperature increase range from 1.8 to 4.0 degrees Celsius according to the different emission scenarios. As for the regional effects, the projections show with greater certainty than ever that the expected changes will mirror the patterns of change observed over the past 50 years (eg, more warming over land) but the rate of change will be higher than before.
The simulations also imply that the removal of excess carbon dioxide from the atmosphere through natural processes on land and sea will become less effective as the Earth warms. Therefore, a larger percentage of the carbon dioxide emitted will remain in the atmosphere and further increase global warming. This is an important positive feedback in the carbon cycle (the changes that carbon compounds undergo in the climate system). Although the models all show that the changes in the carbon cycle will result in a positive feedback, the range of their response remains wide due to other factors, such as changes in vegetation and the capacity to absorb carbon in the soil as the climate warms. Our knowledge about these factors is lacking, and they serve as an important topic in the researches currently being conducted.
The models also predict that climate change will affect the physical and chemical properties of the oceans. Estimates of sea level rise during the 21st century range from 30 to 40 centimeters, again based on emissions. More than 60% of this increase comes from the thermal expansion of the volume of water. However, these calculated estimates do not take into account possible acceleration due to the recently observed melting of the ice sheets in Greenland and Antarctica. Although scientists do not fully understand these factors, the thaw could add another 10 to 20 centimeters to rising sea levels, and even larger rises cannot be ruled out. The chemistry of the water is also affected because as the concentration of carbon dioxide dissolved in the water rises, the water becomes more acidic.
Some of the biggest changes are predicted in the polar regions. Among other things, this is the increase in the temperature of the earth in the high latitudes, the deepening of the thaw in the frozen regions and a sharp reduction in the sea ice cover in the summer in the Arctic basin. At lower latitudes, more heat waves will occur, heavier precipitation will fall, and hurricanes and typhoons will be more intense (though perhaps less frequent). The rate of strengthening of hurricanes and typhoons is uncertain, and the issue is now at the center of many studies.
Of course, some important uncertainties still remain. For example, the exact way the clouds react to the rising temperatures is a decisive factor affecting the degree of expected overall warming. But the complexity of the clouds makes it frustratingly difficult to make such an accurate determination, and here too much more research is needed.
We live today in a time when both humans and nature influence the future development of the earth and its inhabitants. Unfortunately, the crystal ball of climate models gets clouded when it comes to predicting what will happen more than 100 years from now. Our uncertainty stems from the limited knowledge we have about the response of the natural system and human society to the effects of climate change. But one consequence of global warming is certain. The plants, animals and humans will have to live with the consequences of climate change for at least another 1000 years.
key concepts
Scientists are sure that humans have affected the climate, and that the climate change caused by humanity is expected to continue in the future. Greenhouse gases emitted due to human activity, and especially the burning of mineral fuel, are the main driving force in the recent climate change. The report of the Intergovernmental Panel on Climate Change (IPCC) stated that the probability that human activity is the cause of global warming is higher than 90%. The previous report, published in 2001, stated that the probability of this is only 66%. Although continued change in the world's climate is now inevitable, control over the future, especially in the long term, still remains largely in our hands - the expected rate of change depends on what humans choose to do about greenhouse gas emissions.
Glossary of Terms
* Radiative forcing, in the sense used in the text box on the opposite page, is the change in the Earth's energy balance from the pre-industrial era to today.
* Long-lived greenhouse gases include carbon dioxide, methane, nitrogen oxides and halogenated carbon compounds (compounds in which carbon atoms are bonded to atoms of the halogen elements, fluorine, chlorine, bromine or iodine). The observed increases in the concentration of these gases are a result of human activity.
* Ozone is a gas found both in the upper layers of the Earth's atmosphere and at ground level. In the lower layers the gas is an air pollutant, while in the upper atmosphere the ozone layer protects life on earth from harmful ultraviolet radiation from the sun.
* Surface albedo is the degree to which the Earth's surface reflects light: a lighter surface, such as snow cover, reflects more solar radiation than dark areas.
* Aerosols are airborne particles of natural origin (sandstorms, forest fires, volcanic eruptions) or artificial, such as the burning of mineral fuel.
* Airplane trails are artificial clouds of condensation caused by exhaust gases from the engine.
* The troposphere is the layer of the atmosphere closest to the ground. It rises to a height of about 12 kilometers above sea level.
* The stratosphere is the next layer above the troposphere and it reaches a height of about 50 kilometers.
Factors affecting climate
In the hands-down competition between the positive constraints (which cause the climate to warm) and the negative constraints (which cause it to cool), the forces that originate mainly from humans and that bring about warming are winning. The main constraints that man causes are mainly due to long-lived greenhouse gases in the atmosphere, whose concentrations have risen over the past 200 years.
The observed evidence
Observations that include the global average of surface temperatures, sea level and the area covered by snow in March and April in the Northern Hemisphere of the Earth, document increasing warming. The red lines show the average values over a decade, the blue shading shows the range of uncertainty. The blue dots show annual values. All values are calculated relative to the average of 1990-1961.
Temperature change due to human influence
Models that consider only natural constraints do not reflect the real increase in temperature. But when including both natural and man-made constraints, the models reproduce the increase in temperature that occurred in reality, both on a global scale and on a continental scale. The changes shown here are calculated relative to the average of the years 1950-1901.
the authors
The authors are members of Working Group No. 1 that prepared the 2007 IPCC assessment. William Collins (Collins) is an associate professor in the Department of Earth and Planetary Sciences at the University of California, Berkeley and a senior scientist at the US Lawrence Berkeley National Laboratory and the US National Center for Atmospheric Research in Boulder, Colorado. Robert Colman is a senior researcher in the Climate Dynamics Group at the Center for Meteorological Research in Melbourne, Australia. James Haywood is Director of Aerosol Research in the Observational Research Group and the Chemistry, Climate and Ecosystems Group at the Met Office in Exeter, England. Martin R. Manning is the Director of the IPCC Working Group 1 Support Unit at the Earth System Research Laboratory at the US Meteorological Service in Boulder, Colorado. Philip Mote is the Washington State Climatologist, a research scientist in the Climate Impacts Research Group at the University of Washington and an adjunct professor in the Department of Atmospheric Sciences.
The predicted temperature changes
The predicted changes in ground level temperatures (relative to the years 1999-1980), based on 22 computer models from 17 research programs, were calculated for three socio-economic scenarios. All three are based on studies conducted before the year 2000 and they start from the assumption that the climate policy has not changed, meaning they do not include actions to mitigate the warming.
IPCC Mechanism of Action
The Intergovernmental Panel on Climate Change, IPCC, was founded by governments in 1988 to assess the available scientific and technical information on climate change. The evaluation process is designed to create a high level of credibility both in the scientific community and among policy makers. Comprehensive sets were published in 1990, 1995, 2001 and 2007.
Three separate "working groups" examine the physical basis of climate change, its effects on nature and society and the methods to mitigate it.
The governments appoint the lead authors, all of whom are involved in active relevant research. Attention is paid to the balance between different opinions, geographical areas, gender and age.
A peer review process checks the authors' assessments against the opinions of a wider expert community. More than 600 expert reviewers made 30,000 comments on the Working Group 1 report on which this article is based.
Each of the three working groups also issues a "summary for policymakers", written in collaboration with government representatives to ensure that the language is comprehensible to policymakers.
And more on the subject
The reports and summaries of the Intergovernmental Panel on Climate Change: www.ipcc.ch
on the science site
