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
Celsius (58.1 degrees Fahrenheit), this decade was 0.2 degrees Celsius
(0.36 degrees Fahrenheit) 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 Celsius (1.4
degrees Fahrenheit) 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 Celsius (3.6–5.4
degrees Fahrenheit) 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 Celsius (2–11 degrees Fahrenheit) 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.
