The year 2007 tied with 1998 as the second warmest year on record, with an average global temperature of 14.57 degrees Celsius, according to NASA's Goddard Institute for Space Studies.  The average global temperature in 2007 was nearly 0.6 degrees Celsius greater than the average between 1951 and 1980 and more than 0.8 degrees Celsius above the average recorded from 1881 to 1910.  The World Meteorological Association ranks 1998–2007 as the warmest decade on record.

That 2007 was so warm is particularly significant because throughout the year important cooling influences prevailed. These included low solar irradiance (the energy Earth receives from the Sun) and a strong La Niña in the Pacific. These natural processes were counter­acted by the build-up of greenhouse gases caused principally by the combustion of fossil fuels, with other important contributions from agriculture, land use change, and industrial gases.   In 2007, the concentration of atmospheric carbon dioxide (CO2) climbed to a new high of 383.6 parts per million.  (See Figure 3.)

The Intergovernmental Panel on Climate Change (IPCC) released its Fourth Assessment Report in 2007, in which it concluded with greater than 90 percent certainty that emissions of CO2 and other greenhouse gases from human activities are driving climate change.  The report, which represents the work of thousands of experts and scientists, describes a litany of impacts to natural and managed systems that are already happening or are likely to occur if we continue with business as usual.

Even if emissions stopped rising today, additional warming is inevitable due to the large inertia in the climate system. CO2 persists in the atmosphere for 50–200 years, which means that current emissions will exert a warming influence for decades to come.  Meanwhile, the ocean, which acts as a vast heat sink, will continue to warm. As it does, air temperatures will likely rise to double the warming already witnessed.

Current trends suggest that we may experience even more than this amount of warming. As anthropogenic emissions are rising, the effici­ency of natural carbon sinks is in decline. A 2007 study led by the Global Carbon Project concluded with high confidence that the share of emissions absorbed by ocean and terrestrial sinks is falling; this, in turn, is accelerating the rise in atmospheric CO2 concentration beyond the rate of emissions increase.

While climate change is a global challenge, many global indicators—like average temperature—overlook the dramatic changes occurring at regional and local levels. The World Meteorological Organization reports that parts of Europe experienced winter and spring temperatures more than 4 degrees Celsius above average in 2007, and extreme drought struck North America and China.  Massive floods caused devastation in England, South Asia, and many South American countries.  While causal links cannot be made between climate change and specific weather events, more extreme weather is consistent with expectations for a warmer globe.

Warming in the northern hemisphere is more pronounced than the global average. Much of the Arctic experienced an average 2007 temperature that was greater than 2 degrees Celsius above the 1951–80 mean. Arctic sea ice coverage reached a record low by September 1 (summer's end)—39 percent below the September 1 average over the 1979–2000 period and 23 percent below the coverage just two years earlier, in 2005—prompting scientists to predict a complete disappearance of summer sea ice by 2030. Loss of sea ice creates a positive feed­back in the climate system, as open water absorbs far more solar energy than ice and snow do, driving further warming.

Land-based ice melt is also increasing, with serious implications for coastal communities, wildlife, and ecosystems. Two major glaciers in southeast Greenland have lost approximately 122 cubic kilometers of ice each year since 2001, and scientists estimate that Greenland's contribution to sea level rise is now about 0.6 milli­meters annually. A recent study of Ant­arc­tica concluded that the continent is also experiencing a net ice loss and that the pace of ice melt accelerated 75 percent over the past decade. According to the IPCC, land-based ice melt and thermal expansion caused sea levels to rise 3 millimeters per year between 1993 and 2003.

The rapid pace of global environmental change is likely to exceed the capacities of many species to adapt. Coral reefs—vital in natural, cultural, and economic terms—appear particularly vulnerable to even the most modest cli­mate change scenarios, as they are unable to adapt to rapid changes in temperature and ocean acidity. A 2007 report to the U.S. Congress concluded that while some species could thrive in response to projected climate change, impacts on many others "may include extinc­tions, changes in species' ranges, mismatches in their phenologies (timing of pollination, flowering, etc.), and population declines." Further, "climate change acts in concert with other variables to effect changes in species," according to several studies cited in the report, and it is uncertain how wildlife will adapt.

All this new information brought a sense of heightened urgency to the global discourse on climate change in 2007. In April, for the first time, the U.N. Security Council took up the issue of climate change and its potential impacts on peace and security. Climate change was also addressed at the June meeting of the G8 in Germany, at a high-level U.N. summit in New York in September, and at a U.S.-hosted gathering of the world's major economies, also in September. The year concluded with the 192 parties to the U.N. Framework Convention on Climate Change agreeing in Bali to negotiate by 2009 a new global climate pact that will include plans for mitigation, adaptation, technology transfer, and financing.

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