Looking at the northern hemisphere alone, 2007 temperatures averaged 15.04 degrees Celsius (59.1 degrees Fahrenheit)—easily the hottest year in the northern half of the globe since the record began in 1880, and more than a degree warmer than the 1951–80 average. Paleo-temperature records from ancient tree rings suggest that the northern hemisphere is now warmer than at any time in at least the last 1,200 years.
The year 2007 fits into a pattern of steadily increasing global average temperature, with the eight warmest years on record all occurring in the last decade. According to the dataset maintained by NASA’s Goddard Institute for Space Studies, global average temperature rose from 14.02 degrees Celsius in the 1970s to 14.26 degrees in the 1980s and then to 14.40 degrees in the 1990s. In the first eight years of the twenty-first century, the world averaged 14.64 degrees Celsius. (See data.) Since 1990, mean global temperature has risen by 0.33 degrees, a rate of increase faster than climate models had predicted.
Although 2007 did not post a new record high, the year stands out as being extremely warm despite several natural factors that usually cool the planet. El Niño conditions in the southern Pacific tend to increase the global average temperature, and yet the second half of 2007 saw the opposite develop—a La Niña, which would usually depress global temperature. This is in stark contrast to conditions in 1998, the third warmest year, when temperatures were boosted around 0.2 degrees Celsius by the strongest El Niño of the century. In addition to the moderate La Niña, solar intensity in 2007 was slightly lower than average because the year was a minimum in the 11-year solar sunspot cycle. The combination of these factors would normally produce cooler temperatures, yet 2007 was still one of the warmest years in human history. This strongly suggests that the warming effect of increased greenhouse gas concentrations is now dwarfing other influences on the Earth’s climate.
The impact of the exceptional warmth in 2007 was especially apparent in the Arctic, where several feedback mechanisms amplify the effect of increasing greenhouse gas concentrations. Summer sea-ice extent in the Arctic Ocean shrank dramatically to a new low, 23 percent below the previous 2005 record. This opened the Northwest Passage for the first time in recorded history and prompted a scramble to claim large swaths of the newly exposed Arctic.
Regionally, several areas saw record-setting temperatures in 2007. Southeastern Europe suffered through temperatures as high as 45 degrees Celsius in a heat wave that killed up to 500 people. In Japan, thermometers in August reached 40.9 degrees Celsius, the highest temperature ever recorded in that country. Chart-topping temperatures and severe drought conditions proved a lethal combination, as extensive wildfires spread in both Greece and the American West in July and August.
While some areas baked under intensive heat or drought conditions, others were inundated by record amounts of rain. England and Wales experienced widespread flooding and damage estimated at £3 billion ($6 billion) during the wettest May to July period since records began in 1766. In South Asia, some of the worst flooding in decades occurred during the monsoon season, affecting at least 25 million people and killing more than 2,500. Fifteen countries across Africa—from Ghana to Ethiopia—were affected by severe floods in the summer of 2007. These displaced hundreds of thousands of people and washed away crops and farmland, seriously damaging food security in the region. Other countries that saw exceptional or record flooding in 2007 include China, Indonesia, Mexico, and Uruguay.
Intense rainfall events and flooding will only become more common in the future, as climate models show that warmer temperatures will cause a greater share of total precipitation to fall in extreme events. This means that there will be more heavy rainstorms but also more dry periods, producing both more severe droughts and more frequent, more intense floods. Rainfall data from the twentieth century show precipitation intensity increasing over the last two decades, suggesting this trend is already beginning.
In 2007, the Intergovernmental Panel on Climate Change (IPCC), the Nobel prize–winning body of more than 1,250 scientists, released its Fourth Assessment Report, which detailed the likely climatic consequences if human beings continue to pump greenhouse gases into the atmosphere. It reported that unabated emissions would result in a temperature rise of between 1.1 and 6.4 degrees Celsius (2 to 12 degrees Fahrenheit) during the twenty-first century.
To put this in perspective, temperatures over the last 100 years rose by a comparably small 0.74 degrees Celsius, and yet this appears to have already contributed to trends of more heat waves, longer and more intense droughts, higher sea level, more frequent heavy rain events, and stronger hurricanes. Future warming on the scale projected by the IPCC will bring with it a multitude of outcomes that can only be described as disastrous. These include hundreds of millions of people exposed to increased water stress, a third of species at increasing risk of extinction, widespread coral mortality, grain yield declines at low latitudes, the loss of 30 percent of remaining coastal wetlands, and the disappearance of glaciers feed some of the world’s major rivers.
The temperature record for 2007 shows that we have now fully entered into what some are calling a new geological epoch, the Anthropocene, in which human activities are the main driver of the global climate system. The many effects of warmer temperature, which we are already beginning to see, will only become more severe and more costly to society if greenhouse gas emissions are not cut quickly and dramatically. Our future now depends on what we do to limit warming by moving away from climate-disrupting fossil fuels and toward renewable energy and energy-efficient technologies.
ADDITIONAL DATAAverage Global Temperature, 1880-2007 figure table
Average Global Temperature by Decade, 1880-2007
Atmospheric Concentration of Carbon Dioxide, 1000-2007
Average Global Temperature, 1880-2007, with Projections to 2100
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