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The first decade of the twenty-first century was the hottest since recordkeeping began in 1880. With an average global temperature of 14.52 degrees C (58.1 degrees F), this decade was 0.2 degrees C (0.36 degrees F) warmer than any previous decade. The year 2005 was the hottest on record, while 2007 and 2009 tied for second hottest. In fact, 9 of the 10 warmest years on record occurred in the past decade.

Temperature rise has accelerated in recent decades. The earth’s temperature is now 0.8 degrees C (1.4 degrees F) higher than it was in the first decade of the twentieth century, and two-thirds of that increase has taken place since 1970.

Even with these seemingly small increases in global temperature, natural systems are already starting to respond, as evidenced by melting ice sheets and glaciers, shifting weather patterns, and changes in the timing of seasonal events. If temperatures continue to rise on their current trajectory, by the end of the century they will have left the narrow range in which human civilization has developed and flourished.

Though temperatures are rising around the globe, some areas are warming faster than others, with the greatest warming taking place in the Arctic. Paleoclimate records from Arctic lakes, tree rings, and ice cores reveal that the past decade was the warmest of the past two millennia. Warming is amplified in the Arctic for a number of reasons, including the loss of the region’s extensive snow and ice cover: as temperatures rise and light-reflecting ice melts, it is replaced by darker water, which absorbs more energy from the sun, thereby accelerating warming. In parts of the Arctic, average annual temperatures have increased by as much as 2–3 degrees C (3.6–5.4 degrees F) since the 1950s. In 2007, Arctic summer sea ice shrank to its lowest extent on record, leaving the Northwest Passage completely ice-free for the first time in human memory. Then 2008 and 2009 brought the second and third lowest extent of Arctic summer ice on record.

The earth’s temperature is determined by a number of factors. One major influence is the El Niño–Southern Oscillation (ENSO). This cycle, which involves large shifts in atmospheric and ocean temperatures over the tropical Pacific, has two phases: El Niño, which typically raises average global temperature, and La Niña, which lowers it. Year-to-year temperature variations are also influenced by the amount of energy the earth receives from the sun: increases in solar activity tend to raise global temperatures, while decreases in solar activity lower them.

These natural cycles alone, however, fail to explain the temperature patterns of the last decade. While the strongest El Niño of the century pushed 1998 temperatures up to their then-record high, temperatures in the hottest year (2005) did not receive a boost from El Niño. And 2007 was tied for second hottest year on record, despite the development of a cooling La Niña. Furthermore, while global temperatures have been climbing to record heights, incoming solar energy has in fact been declining since the beginning of the decade. In early 2009, solar activity reached its lowest level in a century.

Rather than ENSO cycles or variations in solar irradiance, human-induced warming from heat-trapping greenhouse gases has become the dominant climate influence. Carbon dioxide levels in the atmosphere have risen rapidly since the start of the Industrial Revolution, climbing from 280 parts per million (ppm) in the late eighteenth century to 387 ppm today. Researchers recently reported that the last time atmospheric carbon dioxide levels were this high was roughly 15 million years ago, when sea level was 25–40 meters (80 to 130 feet) higher, and temperatures were approximately 3–6 degrees Celsius warmer.

The risks posed by rising global temperature are widespread. As the atmosphere warms, mountain glaciers that provide water to over a billion people are melting. Melting ice sheets and thermal expansion of oceans raise sea levels, threatening coastal populations. Increasing temperatures bring decreasing crop yields, putting world food supplies at risk. And ecosystems worldwide are irrevocably altered, placing large numbers of species at risk of extinction.

Higher global temperatures also bring with them more frequent and severe extreme weather events. Over the past few decades, scientists have noted an increase in hot extremes and a decrease in cold extremes across the globe. As temperatures rise further, heat waves will become more frequent and intense. Longer and more severe droughts will take place over wider areas; an upsurge in global drought since the 1970s, associated with higher temperatures, has already been observed. At the same time, as temperatures rise, the water-holding capacity of the atmosphere increases, leading to more intense storms and flooding in areas that are already wet.

The past decade saw many record-breaking extreme weather events, providing examples of the kinds of incidents expected to become more frequent with global warming. In the summer of 2003, Europe experienced an intense heat wave that led to over 52,000 deaths. In the United States, where daily record high temperatures occurred twice as often as record lows over the last 10 years, persistent drought plagued parts of the South and West for much of the second half of the decade. A 2006 heat wave affecting the West and Midwest was blamed for 140 deaths in California.

The combination of high temperatures and drought makes a dangerous recipe for wildfire; indeed, 2006 and 2007 saw the worst fire seasons on record in the United States. A similar combination led to disaster in southeastern Australia in early 2009: on what is now known as Black Saturday, intense, rapidly spreading bushfires killed 173 people and burned over a million acres.

Other areas have experienced unusually heavy rains and flooding over the past decade. Record flooding hit Central Europe in 2002, causing over 100 deaths and forcing 450,000 people to evacuate. In summer 2007, the worst flooding in 60 years in England and Wales killed nine people and caused billions of dollars worth of damage; that May to July period was the wettest in the region since recordkeeping began in 1766. In 2008, extensive flooding occurred in several parts of the African continent; Algeria saw its worst floods in a century, while Zimbabwe’s floods were its worst on record.

As temperatures rise, warmer oceans provide more energy to feed tropical storms. The past few decades have seen an increase in the frequency of the most severe hurricanes, and researchers have identified rising sea surface temperatures as the primary cause. The 2005 Atlantic hurricane season was the worst on record, with 27 named storms, 15 of which were classified as hurricanes — including Hurricane Katrina, which caused over 1,300 deaths and $125 billion in financial losses.

In 2007, the Intergovernmental Panel on Climate Change (IPCC), an international body of over 2,500 scientists, released its Fourth Assessment Report, in which it called the recent warming of the globe “unequivocal.” The report projected a rise in average global temperature of 1.1–6.4 degrees C (2–11 degrees F) by the end of the century. Based on the most recent scientific assessments, if greenhouse gas emissions continue to grow at their current pace, the temperature rise by the end of the century will likely reach or exceed the upper end of these projections. Already, effects of increasing temperatures such as accelerating ice melt and sea level rise are outpacing the IPCC’s predictions of just three years ago. Without significant cuts in greenhouse gas emissions, global temperature will rise dramatically by the end of the century, creating a world that looks vastly different from the one we know today.

Data and additional resources available at www.earthpolicy.org.