2005 Hottest Year on Record

Joseph Florence * Earth Policy Institute March 28, 2006

The year 2005 was the hottest on record.  The average global surface temperature of 14.77 degrees Celsius (58.6 degrees Fahrenheit) was the highest since recordkeeping began in 1880.  January, April, September, and October of 2005 were the hottest of those months on record, while March, June, and November were the second warmest ever.

Figure 1.

 

In fact, the six hottest years on record have all occurred in the last eight years.  After 2005, 1998 was the second warmest, with an average global temperature of 14.71 degrees Celsius.  But there was an important difference between 1998 and 2005: the strongest El Niño of the past 100 years lifted the average 1998 temperature 0.2 degrees Celsius, whereas the record warmth last year was not buoyed by such an effect.

These readings, which come from the series maintained by NASA’s Goddard Institute for Space Studies, continue a trend of rising global temperatures. During the past century, temperatures rose 0.8 degrees Celsius (1.44 degrees Fahrenheit), 0.6 degrees of which occurred during the last three decades, a rate unprecedented in the last millennium. The average temperature of 14.02 degrees Celsius in the 1970s rose to 14.26 degrees in the 1980s. In the 1990s it reached 14.40 degrees Celsius. And during the first six years of this new decade, global temperature has averaged 14.62 degrees Celsius.

Table 1.

Average Global Temperature by Decade, 1880-2005
Decade	Average Temperature
	Degrees Celsius
1880-1889	13.82
1890-1899	13.69
1900-1909	13.74
1910-1919	13.79
1920-1929	13.91
1930-1939	14.02
1940-1949	14.05
1950-1959	13.98
1960-1969	13.94
1970-1979	14.02
1980-1989	14.26
1990-1999	14.40
2000-2005	14.62

Rising temperatures are due primarily to the buildup of greenhouse gases in the atmosphere, particularly carbon dioxide (CO2) from the burning of fossil fuels. Once released into the atmosphere, CO2 traps heat that would otherwise escape back into space. Emissions of CO2 have been rising since the start of the Industrial Revolution in 1760, causing temperatures to climb.

Figure 2.

Two recent reports demonstrate the exceptional levels of current global temperature and atmospheric CO2. Using records stored in ice, tree rings, and fossils, scientists have estimated that the northern hemisphere is warmer now than at any time in the past 1,200 years. Another study reported that atmospheric levels of CO2 and methane, another greenhouse gas, are higher today than at any time in the last 650,000 years.

As greenhouse gas emissions continue to increase, so too will the pace of climate change. By 2100 the average global temperature is projected to rise 1.4‑5.8 degrees Celsius relative to the 1990 level, according to the Intergovernmental Panel on Climate Change, a global body of more than 1,500 scientists.

(See Figure 3.)

There is little question that a global temperature increase in the upper range of predictions would be highly disruptive. As global temperatures continue to rise, so do the health risks from heat waves, failing crops, infectious diseases, and other environmental changes. People already facing food insecurity could be particularly distressed because with each 1-degree-Celsius increase in temperature above optimal levels, wheat, rice, and corn yields fall by 10 percent. Even at the lowest projected temperature increases, climate change models predict more frequent and more severe storms, floods, heat waves, and droughts—all of which would affect biodiversity, human health, and economic security.

Such effects may have already begun to occur. In 2005, for example, parts of Brazil’s Amazon rainforest experienced the worst drought in over a century, thought to have been precipitated in part by abnormally high temperatures in the North Atlantic Ocean. With less rainfall, tropical forests dry out and burn more easily. Less moisture evaporates from these drier forests, leading to less precipitation, which perpetuates the problem.

The rise in sea surface temperature has also contributed to a record-breaking Atlantic hurricane season, with 27 named storms and 15 hurricanes in 2005. In August, Hurricane Katrina ravaged the U.S. Gulf Coast, leading to more than 1,100 deaths and displacing approximately 1 million people. The United States suffered an estimated $75 billion in damage due to the hurricane—the costliest natural disaster in U.S. history.

A report by the Pew Center on Global Climate Change analyzing the results of 40 previous studies found a clear link between increased temperatures and numerous changes in natural systems across the United States. Warmer winters, increased precipitation, and earlier springs are causing certain plant species to bloom several weeks earlier, which is disrupting insect food supplies and plant pollination cycles. Temperature changes have led to shifts in many species’ habitats as populations move north and to higher elevations in search of cooler temperatures. Scientists estimate that about half of all wild species in the United States have already been affected by climate change.

Warming in the Arctic—the area around the North Pole, including parts of Russia, Alaska, Canada, Greenland, and Scandinavia—has occurred at nearly twice the global average rate. Indeed, a snapshot of 2005 shows that the greatest warming last year occurred in the Arctic Circle.  Warming there is enhanced by a positive feedback mechanism. Snow and ice reflect some 80 percent of solar radiation. When they melt, more heat is absorbed by the underlying surface, which in turn melts more snow and ice. From 2002-2005, summer Arctic sea ice has covered 20 percent less area than its 1978-2000 summer average. The Arctic could be ice-free in summers by the end of this century, threatening the fate of the polar bear as melting ice shrinks its habitat and compromises access to food.

In addition, in Western Siberia, an area of permafrost spanning a million square kilometers—the size of France and Germany combined—has recently begun to melt for the first time since it was formed over 11,000 years ago at the end of the last ice age. This permafrost covers the world’s largest frozen peat bog. Scientists warn that if warming trends continue it will release billions of tons of stored carbon into the atmosphere, accelerating global warming.

The temperature data for 2005 provide further evidence of what some scientists are calling a new geological epoch, the Anthropocene, in which human activities are the main driver of the global climate system. The amount that temperatures increase depends on what we do from now on to curb emissions of CO2 and other greenhouse gases. We can continue to use climate-disrupting fossil fuels or we can choose to shift to renewable energy sources and more-energy-efficient technologies.

*This is an update of a 9 December 2004 Indicator written by Lila Buckley.

 

Source: Goddard Institute for Space Studies, NASA Goddard Space Flight Center, Earth Sciences Directorate, "Global Temperature Anomalies in .01 C," http://data.giss.nasa.gov/gistemp/tabledata/GLB.Ts.txt, updated March 2006.