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Unit 12: Earth's Changing Climate // Section 6: Present Warming and the Role of CO2


There is clear evidence from many sources that the planet is heating up today and that the pace of warming may be increasing. Earth has been in a relatively warm interglacial phase, called the Holocene Period, since the last ice age ended roughly 10,000 years ago. Over the past thousand years average global temperatures have varied by less than one degree—even during the so-called "Little Ice Age," a cool phase from the mid-fourteenth through the mid-nineteenth centuries, during which Europe and North America experienced bitterly cold winters and widespread crop failures.

Over the past 150 years, however, global average surface temperatures have risen, increasing by 0.6°C +/- 0.2°C during the 20th century. This increase is unusual because of its magnitude and the rate at which it has taken place. Nearly every region of the globe has experienced some degree of warming in recent decades, with the largest effects at high latitudes in the Northern Hemisphere. In Alaska, for example, temperatures have risen three times faster than the global average over the past 30 years. The 1990s were the warmest decade of the 20th century, with 1998 the hottest year since instrumental record-keeping began a century ago, and the ten warmest years on record have all occurred since 1990 (Fig. 9).

Global temperature record

Figure 9. Global temperature record
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Source: Courtesy Phil Jones. © Climactic Research Unit, University of East Anglia and the U.K. Met Office Hadley Centre.

As temperatures rise, snow cover, sea ice, and mountain glaciers are melting. One piece of evidence for a warming world is the fact that tropical glaciers are melting around the globe. Temperatures at high altitudes near the equator are very stable and do not usually fluctuate much between summer and winter, so the fact that glaciers are retreating in areas like Tanzania, Peru, Bolivia, and Tibet indicates that temperatures are rising worldwide. Ice core samples from these glaciers show that this level of melting has not occurred for thousands of years and therefore is not part of any natural cycle of climate variability. Paleoclimatologist Lonnie Thompson of Ohio State University, who has studied tropical glaciers in South America, Asia, and Africa, predicts that glaciers will disappear from Kilimanjaro in Tanzania and Quelccaya in Peru by 2020.

“The fact that every tropical glacier is retreating is our warning that the system is changing.”

Lonnie Thompson, Ohio State University

Rising global temperatures are raising sea levels due to melting ice and thermal expansion of warming ocean waters. Global average sea levels rose between 0.12 and 0.22 meters during the 20th century, and global ocean heat content increased. Scientists also believe that rising temperatures are altering precipitation patterns in many parts of the Northern Hemisphere (footnote 7).

Because the climate system involves complex interactions between oceans, ecosystems, and the atmosphere, scientists have been working for several decades to develop and refine General Circulation Models (also known as Global Climate Models), or GCMs, highly detailed models typically run on supercomputers that simulate how changes in specific parameters alter larger climate patterns. The largest and most complex type of GCMs are coupled atmosphere-ocean models, which link together three-dimensional models of the atmosphere and the ocean to study how these systems impact each other. Organizations operating GCMs include the National Aeronautic and Space Administration (NASA)'s Goddard Institute for Space Studies and the United Kingdom's Hadley Centre for Climate Prediction and Research (Fig. 10).

Hadley Centre GCM projection

Figure 10. Hadley Centre GCM projection
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Source: © Crown copyright 2006, data supplied by the Met Office.

Researchers constantly refine GCMs as they learn more about specific components that feed into the models, such as conditions under which clouds form or how various types of aerosols scatter light. However, predictions of future climate change by existing models have a high degree of uncertainty because no scientists have ever observed atmospheric CO2 concentrations at today's levels.

Modeling climate trends is complicated because the climate system contains numerous feedbacks that can either magnify or constrain trends. For example, frozen tundra contains ancient carbon and methane deposits; warmer temperatures may create a positive feedback by melting frozen ground and releasing CO2 and methane, which cause further warming. Conversely, rising temperatures that increase cloud formation and thereby reduce the amount of incoming solar radiation represent a negative feedback. One source of uncertainty in climate modeling is the possibility that the climate system may contain feedbacks that have not yet been observed and therefore are not represented in existing GCMs.

Scientific evidence, including modeling results, indicates that rising atmospheric concentrations of CO2 and other GHGs from human activity are driving the current warming trend. As the previous sections showed, prior to the industrial era atmospheric CO2 concentrations had not risen above 300 parts per million for several hundred thousand years. But since the mid-18th century CO2 levels have risen steadily.

In 2007 the Intergovernmental Panel on Climate Change (IPCC), an international organization of climate experts created in 1988 to assess evidence of climate change and make recommendations to national governments, reported that CO2 levels had increased from about 280 ppm before the industrial era to 379 ppm in 2005. The present CO2 concentration is higher than any levels over at least the past 420,000 years and is likely the highest level in the past 20 million years. During the same time span, atmospheric methane concentrations rose from 715 parts per billion (ppb) to 1,774 ppb and N2O concentrations increased from 270 ppb to 319 ppb (footnote 8).

Do these rising GHG concentrations explain the unprecedented warming that has taken place over the past century? To answer this question scientists have used climate models to simulate climate responses to natural and anthropogenic forcings. The best matches between predicted and observed temperature trends occur when these studies simulate both natural forcings (such as variations in solar radiation levels and volcanic eruptions) and anthropogenic forcings (GHG and aerosol emissions) (Fig. 11). Taking these findings and the strength of various forcings into account, the IPCC stated in 2007 that Earth's climate was unequivocally warming and that most of the warming observed since the mid-20th century was "very likely" (meaning a probability of more than 90 percent) due to the observed increase in anthropogenic GHG emissions (footnote 9).

Comparison between modeled and observations of temperature rise since the year 1860

Figure 11. Comparison between modeled and observations of temperature rise since the year 1860
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Source: © Intergovernmental Panel on Climate Change, Third Assessment Report, 2001. Working Group 1: The Scientific Basis, Figure 1.1.

Aerosol pollutants complicate climate analyses because they make both positive and negative contributions to climate forcing. As discussed in Unit 11, "Atmospheric Pollution," some aerosols such as sulfates and organic carbon reflect solar energy back from the atmosphere into space, causing negative forcing. Others, like black carbon, absorb energy and warm the atmosphere. Aerosols also impact climate indirectly by changing the properties of clouds—for example, serving as nuclei for condensation of cloud particles or making clouds more reflective.

Researchers had trouble explaining why global temperatures cooled for several decades in the mid-20th century until positive and negative forcings from aerosols were integrated into climate models. These calculations and observation of natural events showed that aerosols do offset some fraction of GHG emissions. For example, the 1991 eruption of Mount Pinatubo in the Philippines, which injected 20 million tons of SO2 into the stratosphere, reduced Earth's average surface temperature by up to 1.3°F annually for the following three years (footnote 10).

But cooling from aerosols is temporary because they have short atmospheric residence times. Moreover, aerosol concentrations vary widely by region and sulfate emissions are being reduced in most industrialized countries to address air pollution. Although many questions remain to be answered about how various aerosols are formed and contribute to radiative forcing, they cannot be relied on to offset CO2 emissions in the future.

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