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Unit 12: Earth's Changing Climate // Section 8: Other Potential Near-Term Impacts

In its 2007 assessment report the IPCC projected that global average surface temperatures for the years 2090 to 2099 will rise by 1.1 to 6.4°C over values in 2001 to 2010. The greatest temperature increases will occur over land and at high northern latitudes, with less warming over the southern oceans and the North Atlantic (footnote 16).

This rate of warming, driven primarily by fossil fuel consumption, would be much higher than the changes that were observed in the 20th century and probably unprecedented over at least the past 10,000 years. Based on projections like this, along with field studies of current impacts, scientists forecast many significant effects from global climate change in the next several decades, although much uncertainty remains about where these impacts will be felt worldwide and how severe they will be.

Climate change is likely to alter hydrologic cycles and weather patterns in many ways, such as shifting storm tracks, increasing or reducing annual rainfall from region to region, and producing more extreme weather events such as storms and droughts (Fig. 14). While precipitation trends vary widely over time and area, total precipitation increased during the 20th century over land in high-latitude regions of the Northern Hemisphere and decreased in tropical and subtropical regions (footnote 17).

Flooding in New Orleans after Hurricane Katrina, 2005

Figure 14. Flooding in New Orleans after Hurricane Katrina, 2005
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Source: © National Oceanic and Atmospheric Administration.

Rising temperatures and changing hydrological cycles are likely to have many impacts, although it is hard to predict changes in specific regions—some areas will become wetter and some dryer. Storm tracks may shift, causing accustomed weather patterns to change. These changes may upset natural ecosystems, potentially leading to species losses. They also could reduce agricultural productivity if new temperature and precipitation patterns are less than optimal for major farmed crops (for example, if rainfall drops in the U.S. corn belt). Some plant species may migrate north to more suitable ecosystems—for example, a growing fraction of the sugar maple industry in the northeastern United States is already moving into Canada—but soils and other conditions may not be as appropriate in these new zones.

Some natural systems could benefit from climate change at the same time that others are harmed. Crop yields could increase in mid-latitude regions where temperatures rise moderately, and winter conditions may become more moderate in middle and high latitudes. A few observers argue that rising CO2 levels will produce a beneficial global "greening," but climate change is unlikely to increase overall global productivity. Research by Stanford University ecologist Chris Field indicates that elevated CO2 prevents plants from increasing their growth rates, perhaps by limiting their ability to utilize other components that are essential for growth such as nutrients. This finding suggests that terrestrial ecosystems may take up less carbon in a warming world than they do today, not more.

Undesirable species may also benefit from climate change. Rising temperatures promote the spread of mosquitoes and other infectious disease carriers that flourish in warmer environments or are typically limited by cold winters (Fig. 15). Extreme weather events can create conditions that are favorable for disease outbreaks, such as loss of clean drinking water and sanitation systems. Some vectors are likely to threaten human health, while others can damage forests and agricultural crops.

Infectious diseases affected by climate change

Figure 15. Infectious diseases affected by climate change
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Source: © Climate change 1995, Impacts, adaptations and mitigation of climate change: scientific-technical analyses, working group 2 to the second assessment report of the IPCC, UNEP, and WMO, Cambridge Press University, 1996.

Melting of polar ice caps and glaciers is already widespread and is expected to continue throughout this century. Since the late 1970s Arctic sea ice has decreased by about 20 percent; in the past several years, this ice cover has begun to melt in winter as well as in summer, and some experts predict that the Arctic could be ice-free by 2100. Ice caps and glaciers contain some 30 million cubic kilometers of water, equal to about 2 percent of the volume of the oceans. Further melting of sea ice will drive continued sea-level rise and increase flooding and storm surge levels in coastal regions.

Warmer tropical sea surface temperatures are already increasing the intensity of hurricanes, and this trend may accelerate as ocean temperatures rise (footnote 18). Stronger storms coupled with rising sea levels are expected to increase flooding damage in coastal areas worldwide. Some scientists predict that extreme weather events, such as storms and droughts, may become more pronounced, although this view is controversial. In general, however, shifting atmospheric circulation patterns may deliver "surprises" as weather patterns migrate and people experience types of weather that fall outside their range of experience, such as flooding at a level formerly experienced only every 50 or 100 years.

Human societies may already be suffering harmful impacts from global climate change, although it is important to distinguish climate influences from other socioeconomic factors. For example, financial damages from storms in the United States have risen sharply over the past several decades, a trend that reflects both intensive development in coastal areas and the impact of severe tropical storms in those densely populated regions. Human communities clearly are vulnerable to climate change, especially societies that are heavily dependent on natural resources such as forests, agriculture, and fishing; low-lying regions subject to flooding; water-scarce areas in the subtropics; and communities in areas that are subject to extreme events such as heat episodes and droughts. In general, developed nations have more adaptive capacity than developing countries because wealthier countries have greater economic and technical resources and are less dependent on natural resources for income.

And more drastic changes may lie in store. As discussed above, climate records show that the climate can swing suddenly from one state to another within periods as short as a decade. A 2002 report by the National Research Council warned that as atmospheric GHG concentrations rise, the climate system could reach thresholds that trigger sudden drastic shifts, such as changes in ocean currents or a major increase in floods or hurricanes (footnote 19).

“Just as the slowly increasing pressure of a finger eventually flips a switch and turns on a light, the slow effects of drifting continents or wobbling orbits or changing atmospheric composition may ‘switch’ the climate to a new state.”

Richard B. Alley, Chair
Committee on Abrupt Climate Change,
National Research Council

How much the planet will warm in the next century, and what kind of impacts will result, depends on how high CO2 concentrations rise. In turn, this depends largely on human choices about fossil fuel consumption. Because fossil fuel accounts for 80 percent of global energy use, CO2 levels will continue to rise for at least the next 30 or 40 years, so additional impacts are certain to be felt. This means that it is essential both to mitigate global climate change by reducing CO2 emissions and to adapt to the changes that have already been set in motion. (For more on options for mitigating and adapting to climate change, see Unit 13, "Looking Forward: Our Global Experiment.")

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