Michael MacCracken
Global Climate Change:
A Science Overview

Introduction

The Earth is changing. An astronomer on some remote planet would be amazed at what is happening, and how rapidly. First came humankind’s changing of the Earth’s vegetation cover, altering the Earth’s reflectivity and local weather. Next came depletion of the stratospheric ozone layer and the opening of the "Antarctic Ozone Hole." And over recent decades has come accelerating modification of the composition of the atmosphere and climate change, often referred to as "global warming." Quite clearly, the natural course of Earth’s evolution is being affected. Although the details can get complicated, the science of climate change is quite straightforward. This can be summarized in six key points that rest on fundamental physics that have slowly been solidified since the possibility of such an effect was first raised over one hundred years ago.

1. Human Activities are Changing Atmospheric Composition

A wide variety of observational and analytical evidence provides convincing evidence that human activities are changing the composition of the atmosphere. Observations at locations around the world indicate that the atmospheric composition of carbon dioxide (CO2) has increased by about 20% since careful measurements began in 1957. Concentrations in air bubbles in glacial ice document about a 36% increase in the CO2 concentration over the past few centuries. There are even some suggestions that the clearing of forests by nomadic tribes about 8000 years ago caused an increase in the CO2 concentration above what would have been its natural level. Figure 1 shows a reconstruction of the CO2 concentration for the past 1000 years, indicating the very unusual nature of the recent increase.

Two types of human activities are mainly responsible for the sharp increase. The first is carbon resulting from changes in use of the land, including the clearing of forests, the plowing of the land, and the growing of crops. These changes are currently responsible for the ongoing release of 1-2 billion tonnes of carbon each year, mainly in the form of carbon dioxide. The second, and most important, source of CO2 emission is the combustion of coal, oil, and natural gas. These fuels are together referred to as fossil fuels because they are derived from the fossilized remains of plants and animals. Basically, we are injecting back into the atmosphere carbon that was stored away by natural processes over periods of tens to hundreds of millions of years – all in the time span of a couple of centuries. There is no doubt that it is these human activities that are changing atmospheric composition.

2. Changing Atmospheric Composition Can Warm the Earth

The second key finding from scientific research, and this has been recognized for over 100 years, is that this type of change in the atmospheric composition will warm the planet by enhancing the Earth’s natural greenhouse effect. Common wisdom is that it is the Sun that keeps us warm, and it is true that it is the Sun that is the fundamental source of energy for our planet. However, it is really the atmosphere that keeps us warm – without the atmosphere it is estimated that the Earth would be some 33°C (or 60°F) colder than it is.

The Sun warms the Earth’s surface a bit; then, as a result, the surface, like any warm body, emits infrared radiation. As this radiation tries to escape from the planet, water vapor, CO2, and other gases in the atmosphere absorb it. Roughly 80% of the energy radiated upward by the surface is radiated back to it by the atmosphere, creating the natural greenhouse effect that keeps the world from freezing.

When we add gases such as CO2 and other similarly acting gases and particles to the atmosphere, a higher amount of the energy emitted from the surface is returned to the surface, intensifying the natural greenhouse effect. A doubling of the CO2 concentration will, over several decades, cause a global warming of about 3°C (about 5°F)

Although we all experience large changes in temperature from day to day, season to season and place to place, a few degrees of global warming has the potential to be very significant. For reference, the global average warming from the peak of the last ice age some 20,000 years ago to the present was only about 5°C (8°F), so doubling the CO2 concentration is equivalent to about half of that warming. We already have had more than a 35% increase in the CO2 concentration and projections indicate that a doubling is very likely to occur by the end of the 21st century unless stringent control measures are invoked over coming decades.

3. Human-induced Climate Change is Evident in the Climate Record

In that this increase in CO2 has been going on for 150 years or so, we should be expecting to see the climate changing in response. Indeed, there are an increasing variety of observations indicating that the climate is changing. According to the National Climate Data Center, the annual mean global average temperature increased by about 0.6ºC (about 1ºF) over the 20th century, with both land and ocean temperatures rising markedly over the past few decades.

Observations also indicate that upper ocean waters are warming and that many glaciers around the world are melting. Both of these processes tend to cause sea level to rise, and indeed, this is being observed. There are also many other indicators that the climate is changing. Perhaps most troubling is that shifts in the ranges of various plants and animals are being observed, and the large majority of these changes are consistent with what would be expected as a result of changes in the climate, even more so than other human-induced changes in the landscape.

Determining if these changes are due to human activities is complicated by the fact that other factors are also causing the climate to fluctuate. These other factors include fluctuations in solar radiation, injection of aerosols into the stratosphere by major volcanic eruptions, injection of sulfate and soot aerosols into the atmosphere from combustion of coal, and just the chaotic interaction of the atmosphere with the oceans. Although the limited records do indicate that volcanic eruptions and changes in solar radiation are likely to have played a role in past fluctuations in the climate, these factors have not changed in a way consistent with recent warming over the past several decades, so the recent warming does not appear to be due to these natural factors. On the other hand, warming appears to be evident before the greenhouse gases started rising rapidly, so it may well be natural influences that caused the warming in the early 20th century.

The best explanation of recent changes in temperature (and also of the observed changes in other variables) is that they are due to the combined effects of (1) increasing the concentration of CO2 and other greenhouse gases and (2) increasing the atmospheric loading of the particulate matter that also results from combustion of fossil fuels. Not all indicators are in full accord, however, and, because the case is largely circumstantial, there are, and need to be, ongoing efforts to try to prove otherwise. However, the preponderance of observations and analyses indicate we are undergoing changes in the climate due to human activities.

4. Climate Change to Accelerate during the 21st Century

So, what lies ahead? At present, each year’s fossil fuel use by the 6-plus billion people on the planet is leading to the emission of 6-plus billion tonnes of carbon to the atmosphere. This is an average of one tonne of carbon per person per year. The distribution of the emissions is, however, quite varied. In the underdeveloped countries, the emissions are of order half a tonne per person; in China, the level is approaching 1 tonne per person; in Europe it is about 3 tonnes per person; and in the US and a few other countries, it is over 5 tonnes per person. When the climate change statement by the US Catholic Bishops (2001) spoke about issues of equity, they were referring in part to this unequal use of fossil fuels and, therefore, about the unequal contribution of different peoples to the problem.

For the future, the world population looks likely to increase to at least 8 billion and possibly reach over 10 billion by the end of the century. As shown in Figure 2, most of the future growth in population is expected to occur in the developing world.

In the absence of restraints on fossil fuel use that might arise because of concern about climate change, emissions of CO2 can be expected to rise dramatically. In particular, China and India appear likely to make extensive use of their low-cost coal reserves to provide the energy needed to enhance overall living standards, although problems of air pollution and acid rain may lead to some limits.

The Intergovernmental Panel on Climate Change (IPCC), representing the collective efforts of about 180 countries, has developed scenarios of plausible societal and emission paths. Considering current trends and future possibilities, emissions could increase significantly by the end of the century. The IPCC’s most ambitious scenario envisions virtually all additional energy coming from alternative energy sources; in its most pessimistic, in a climate sense, most of the additional energy would come from coal. The emissions scenario shown in Figure 3, in which the rate of emissions triples, is roughly a mid-range case. Note that most of the emissions increase is projected to occur from countries currently considered to be developing, a point raised by policymakers in the developed countries who seem to forget that because there are several times more people in developing countries. Their per capita levels, which are perhaps a measure of relative equity, are still well below the per capita level in developed countries.

Accepting the IPCC emissions scenarios as representing a plausible range of what could happen, the CO2 concentration by 2100 would be expected to rise from its current level of about 35% above the preindustrial level to between 100% to 300% above its preindustrial level; that is, the CO2 concentration would be roughly 2 to 4 times its preindustrial concentration. The Earth has not experienced such high a CO2 concentration in tens of millions of years, and unless the emissions were to be cut to about 75% below current levels, which will be very difficult given the higher population and the rising overall standard of living, the CO2 concentration in the atmosphere will still be rising into the 22nd century.

So, what will this mean for the climate? Unfortunately, we can’t construct physical models of the Earth in the laboratory to test things out, and there is no simple algebraic way to represent the Earth’s climate. We must rely on theoretical models of the Earth system that are constructed in supercomputers to simulate the world’s atmosphere, oceans, and land. These computerized global climate models attempt to represent the effects of all of the important processes governing the climate system. In general, the models reasonably represent the large-scale, time-averaged behavior of the Earth system, generally reproducing the seasons, the monsoons, and the geographic distribution of the climate. The model simulations do much less well in representing the details of changes in specific locations.

Presuming that future emissions of carbon are within the bounds of IPCC’s scenarios, the models project an increase in the global average temperature of about 2 to 5ºC (about 3 to 10ºF) during the 21st century as compared to an increase of about 0.6ºC (about 1ºF) over the 20th century – so several times as much. The range is about equally a result of uncertainties in how emissions will change and uncertainties in how the climate will respond.

All of the various models developed by groups around the world, each making their own attempt to best match the behavior of the real world, project that the warming will be greater over land areas than over the oceans and greater in mid to high latitudes than in lower latitudes. The US meets both criteria. Over the US, the warming would be more or less like changing the climate of the northern tier of states to the climate of the central tier, and of the central tier to the southern tier. And if you live in the southern tier now, well, plan to spend summers inside, as the heat index will increase substantially.

Associated with the warming, there will be other changes. Periods subject to frost will shorten, and summers will have more unusually hot days. Temperatures will not cool down so much at night. Rainstorms are likely to come with more intensity, with periods of heavy rain increasing in intensity the most. With evaporation occurring more rapidly, drying will occur faster, and so moisture stress will occur more rapidly. Basically, wet periods will be wetter, and drought periods drier. Mountain glaciers and likely the Greenland and West Antarctic ice sheets will be melting back more rapidly, adding water to the oceans; ocean warming, which will cause ocean waters to expand, will also contribute to sea level rise. Much less certainly, there is the possibility that some sort of abrupt change might occur. Continuing on the present path, the world faces unprecedented climatic change and quite possibly some surprises along the way.

5. Climate Change Impacts on the Environment, Natural Resources and People

So, why should we care that the climate is changing? To provide an initial evaluation, hundreds of scientists from around the country, working with local experts, governmental representatives, and the public, participated in the US National Assessment of the potential consequences of climate variability and change. These are summarized for the U.S. as follows:

1. Assuming continued growth in world greenhouse gas emissions, temperatures in the US are projected to rise 5-9ºF (3-5ºC) on average in the next 100 years, although a wider range of outcomes is possible.
2. Climate change and the potential impacts of climate change will vary widely across the nation. [The table on page 9 summarizes these regional impacts.]
3. Many ecosystems are highly vulnerable to the projected rate and magnitude of climate change. A few, such as alpine meadows in the Rocky Mountains and some barrier islands, are likely to disappear entirely in some areas. Others, such as forests of the Southeast, are likely to experience major species shifts or break up into a mosaic of grasslands, woodlands, and forests. The goods and services lost through the disappearance or fragmentation of certain ecosystems are likely to be costly or impossible to replace.
4. Water is an issue in every region, but the nature of the vulnerabilities varies. Drought is an important concern in every region. Floods and water quality are concerns in many regions. Snowpack changes are especially important in the West, Pacific Northwest, and Alaska.
5. At the national level, the agriculture sector is likely to be able to adapt to climate change. Overall, US crop productivity is very likely to increase over the next few decades, but the gains will not be uniform across the nation. Falling prices and competitive pressures are very likely to stress some farmers, while benefiting consumers.
6. Forest productivity is likely to increase over the next several decades in some areas as trees respond to higher carbon dioxide levels. Over the longer term, changes in larger-scale processes such as fire, insects, droughts, and disease will possibly decrease forest productivity. In addition, climate change is likely to cause long-term shifts in forest species, such as sugar maples moving north out of the US.
7. Climate change and the resulting rise in sea level are likely to exacerbate threats to buildings, roads, powerlines, and other infrastructure in climatically sensitive places. For example, infrastructure damage is related to permafrost melting in Alaska, and to sea-level rise and storm surge in low-lying coastal areas.
8. A range of negative health impacts is possible from climate change, but adaptation is likely to help protect much of the US population. Maintaining our nation’s public health and community infrastructure, from water treatment systems to emergency shelters, will be important for minimizing the impacts of water-borne diseases, heat stress, air pollution, extreme weather events, and diseases transmitted by insects, ticks, and rodents.
9. Climate change will very likely magnify the cumulative impacts of other stresses, such as air and water pollution and habitat destruction due to human development patterns. For some systems, such as coral reefs, the combined effects of climate change and other stresses are very likely to exceed a critical threshold, bringing large, possibly irreversible impacts.
10. Significant uncertainties remain in the science underlying regional climate changes and their impacts. Further research would improve understanding and our ability to project societal and ecosystem impacts, and provide the public with additional useful information about options for adaptation. However, it is likely that some aspects and impacts of climate change will be totally unanticipated as complex systems respond to ongoing climate change in unforeseeable ways.

For the rest of the world, the situation is likely even more challenging. This is especially the case for the developing countries with their less diversified economies and scantier resources for moderating and adapting to adverse consequences. In some areas, such as island nations, sea level rise will be most important, causing serious inundation during storms and exacerbating erosion problems; for other countries, the shifting boundaries of moist and dry regions are likely to seriously impact agricultural production; in other areas, the increase in temperature and absolute humidity is likely to make urban living life miserable; and in some areas the most important consequences are likely to arise from the spread of disease vectors and worsened problems of air and water quality. Issues of equity and fairness need to be considered, perhaps even be at the forefront of the public discussion about what to do.

6. Making the Problem Go Away is Difficult

Even if emissions were to be cut to zero, the world is likely to experience as much warming in the 21st century as in the 20th century. This is partly from the continuing effects of the greenhouse gases already emitted and partly as a result of halting the emissions of the sulfur dioxide that creates the light-colored, sun-reflecting haze over and downwind of industrial areas. This further climatic change, however, would not be the most devastating consequence. Because fossil fuels provide roughly 80% of the world’s energy, the world cannot immediately give up this source of energy without causing global economic collapse – a point made often by the major oil and coal companies.

Keeping emission less than double the pre-industrial concentration of CO2 is called for in the UN Framework Convention on Climate Change. Accomplishing that, however, would require that average per capita emissions worldwide remain at about the current level of one tonne of carbon per person, averaged over the 21st century. While there can be growth in energy generation and use per capita above this level by deriving energy from sources other than fossil fuels, coal is at present the least expensive fuel in many developing countries, so getting energy from other sources would require diverting money needed for basic survival needs such as water purification to generation of non-fossil energy. And to accommodate the growth in carbon emissions for those in the developing world while limiting the growth in the atmospheric concentration of CO2, there would need to be sharp cutbacks in the average emission of carbon by those in the developed world. In that the population of the developing world is several times as large as that of the developed world, per capita cutbacks in the developed world would need to be several times as large as the gains of those in the developing world – each of us would need to cut back enough to allow for the gain by several others living in the developing world.

Given present trends and emission levels, the world is on a path to considerably higher emissions than at present – up to 3 to 4 times as much as at present unless there are much more rapid breakthroughs in non-fossil energy generation than has been the case. While life itself would not be threatened, the world would be a very different place. n

Dr. Mike MacCracken is chief scientist for climate change programs at the Climate Institute, Washington, DC (www.climate.org). The views expressed are those of the author, drawing on the findings of major national and international scientific assessment reports that have undergone extensive expert review.


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