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For the past century, scientists, sailors, and interested citizens have monitored temperature at weather stations around the world. More recently, satellites have enabled us to measure temperature in places as remote as the high Arctic and the middle of the ocean. The results are clear: In most parts of the world, mean annual temperature during the decade 1999–2008 was warmer than 1940–1980 averages (Fig. 49.5). In some places, the temperature change has been slight, but in others, especially at high latitudes, the increase has been as much as 2°C.

We can measure CO₂ levels in the atmosphere, and they are increasing. We can measure global temperature, and it is increasing. Is increasing CO₂ responsible for observed temperature changes? To address this question, we must understand that carbon dioxide is a greenhouse gas—that is, a gas that absorbs heat energy and then emits it in all directions. As shown in Fig. 49.6, solar radiation passes freely through the atmosphere, from top to bottom. Some incoming radiation is reflected from Earth’s surface, and the rest is absorbed by the land and sea. In turn, some of the energy absorbed by Earth’s surface is radiated back again as infrared radiation, or heat. Greenhouse gases in the atmosphere absorb the infrared radiation reflected up from Earth’s surface and emit it in all directions—some is directed upward, out of the atmosphere, but half of the trapped heat is directed downward, toward Earth. The net effect is like that of the glass panes of a greenhouse, which allow sunlight into the structure but prevent heat from leaving.

Carbon dioxide is only one of several important greenhouse gases in the atmosphere—water vapor is another, and methane a third—and without these gases absorbing and trapping heat, average surface temperatures would fall below freezing and life would not be possible. However, because CO₂ is increasing rapidly, its greenhouse effect is also increasing. Methane levels are also rising rapidly, in large part because of increasing food production. Most of the methane delivered each year to the atmosphere is generated biologically by methane-producing archaeons (Chapter 26), which thrive in the guts of cattle and in the waterlogged paddies where rice is cultivated. More beef and expanded rice paddies translate into higher rates of methane production. The thawing of permafrost at high latitudes releases additional methane that was trapped in frozen soils when the ice formed long ago.

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Physics can help us determine how much atmospheric warming is due to increases in greenhouse gases. Each molecule of greenhouse gas absorbs and emits a specific amount of heat, and calculations show that the increases in atmospheric greenhouse gases measured over the past 50 years have increased the difference between incoming (solar) and outgoing radiation by about 2.5 watts per square meter. This difference is what adds heat to the oceans and atmosphere. Scientific consensus, reflected in reports from the Intergovernmental Panel on Climate Change, is that this greenhouse effect is the principal cause of observed twentieth-century temperature change. Scientific consensus also holds that global temperature will continue to rise as atmospheric CO₂ continues to increase in this century.

If this view is correct, human activities are changing the world. Can we, however, eliminate the possibility that the observed increases in greenhouse gases and temperature have natural causes? After all, as we saw in Chapter 25, the long-term geologic record indicates that climate and atmospheric composition have changed dramatically and repeatedly throughout our planet’s history. Perhaps volcanic emissions have increased, driving the observed changes. However, measurements of the isotopic composition of atmospheric CO₂ effectively eliminate volcanic eruptions as a principal source of rising CO₂ (see Fig. 25.4).

Moreover, we can monitor the effects of volcanoes as they occur, and we can gauge the effects of past eruptions because volcanic ash accumulates along with the ice in continental glaciers. Historically, the major effect of large volcanic eruptions has been to decrease temperature because volcanic ash and aerosols reflect incoming solar radiation back into space. What’s more, the impact of volcanic eruptions lasts only a few years. It doesn’t drive the century-long temperature increase that we are currently observing.

What about the possibility that the amount of solar radiation entering the atmosphere has varied through time? The sun’s output oscillates, and we know from direct measurements how solar radiation has varied during the past 50 years. The effect of this variation can be calculated, and it is small relative to greenhouse effects. It does not seem that variable solar radiation can account for the amount of temperature change that we have observed. In short, natural processes alone cannot explain the temperature increases observed in recent decades (Fig. 49.7).

As a result, there is overwhelming agreement among scientists that atmospheric CO₂ levels are rising, that temperature is increasing, and that the physics of greenhouse gases relates the two effects. Beyond these facts, however, it is difficult to say with certainty how human-induced global trends in climate will affect a given area.

Why don’t we know for certain? Modeling future climatic conditions is difficult because of the many complex interactions that contribute to climate. Climate models are attempts to understand how climate works by fashioning equations that relate a simplified set of variables and interactions. Using mathematical models that accurately approximate current climate, scientists can change inputs into the model, adding CO₂ to the atmosphere, for example, or changing the extent of forest cover. The model then generates a set of results that can be used to predict future climate. All models must be checked against actual observations, and most are sensitive to assumptions made in constructing the model. That said, climate models do a pretty good job of explaining global-scale features of climate and climatic change.

Nevertheless, questions remain. How, in detail, might cloud cover change over the next century, and what effect would this change have on temperature and precipitation? What will be the effect of additional air pollutants, like the black carbon particles released into the air when coal or wood burns incompletely? Will black carbon increase warming by absorbing solar radiation, or decrease it by reflecting radiation back into space? What will be the quantitative effects of feedbacks such as the release of methane and carbon dioxide as warming temperatures melt permafrost at high latitudes? How will oceanic and atmospheric circulation patterns change, and will any changes that may occur enhance or dampen climatic change?

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We still seek definitive answers for all these questions. Nonetheless, most climate models suggest that mean global temperature will increase 2°C–5°C during the 21st century (Fig. 49.8). That increase is not expected to occur uniformly throughout the globe. As has been true during the past 50 years, changing circulation patterns may cool some places and warm others. Likewise, rainfall is likely to increase in some areas, and decline in others.

As discussed in Chapter 48, climate can be considered as average weather over a long time interval. From one year to the next, however, weather is tremendously variable—there will be cold years and warm years, wet ones and dry ones. Because weather is so variable, evidence for climate change comes not from individual weather events but from records kept over decades. We can draw few conclusions from the single observation that May 2015 was Earth’s warmest May on record, but the fact that May 2015 was the 361st month in a row that was warmer than its twentieth-century average tells us the world is warming.

Models predict that rainfall patterns should change as Earth warms. To test this, oceanographers analyzed 1.7 million measurements of seawater salinity taken over the past century. The oceans were chosen because they contain 97% of our planet’s water and receive 80% of its rainfall. The salinity of surface seawater reflects both the addition of fresh water by rain, which decreases salinity, and evaporation, which increases it. For this reason, changing salinity can indicate whether the balance of rainfall and evaporation is shifting over broad regions of Earth. Consistent with many model predictions, wet areas of Earth are becoming wetter and dry regions drier.

Quick Check 2 What is the difference between global warming and the greenhouse effect?