FIGURE 2.2 The concentration of CO2 in the atmosphere (magenta line and left axis, in parts per million), as measured in ice cores and canisters of air collected from multiple locations around the globe, has risen steady since the mid-19th century, with the sharpest rate of increase occurring over the past few decades. Much of this increase can be attributed to global CO2 emissions due to fossil fuel burning (blue line and right axis), which include estimated emissions from the production, distribution, and consumption of fossil fuels, plus a small contribution from cement production. Changes in land use and land cover—especially deforestation—also contribute to changes in atmospheric CO2 concentrations, with current emissions estimated at 4.4 million metric tons per year (or about 12 percent of total emissions from human sources). SOURCE: NRC, America’s Climate Choices: Advancing the Science of Climate Change ( Washington, D.C.: National Academies Press, 2010). See Figure 2.3 from that report for further references.

global warming and these other changes is difficult: often the regional changes remain within the range of past observed variability, the data are not extensive enough, or the models not sufficiently developed to clearly identify an anthropogenic signal. As a result, only a few changes have been directly linked to human activities using formal scientific attribution methods.13

Among the ongoing changes in the physical climate system14 that can be linked, at least in part, to increasing temperatures at the Earth’s surface are widespread melting of glaciers and ice sheets,15 rising global average sea levels,16 and decreases in Northern Hemisphere snow cover17 and Arctic sea ice.18 These changes have, in turn, been linked to a number of impacts on other physical and biological systems over the past several decades.19 For example, permafrost (permanently frozen ground) is thawing across many regions in the Northern Hemisphere,20 lakes and rivers are freezing later and melting earlier.21 Elevated CO2 levels in the atmosphere are also causing widespread acidification of the world’s oceans, which poses significant risks to ocean ecosystems.22

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