The first decade of the 21st century was the warmest decade on record and yielded some of the most extreme weather events – drought, floods, heat waves, intense rain and snowfall – on record in various regions of the world, according to an overview of the last decade produced by the World Meteorological Organization (WMO).
While natural swings in climate played a key role, those have been superimposed over a general warming trend in Earth's climate. The trigger for this longer-term warming has come from rising carbon-dioxide emissions from burning fossil fuel and from land-use changes.
The report "shows that global warming was significant from 1971 to 2010 and that the decadal rate of increase between 1991-2000 and 2001-2010 was unprecedented," said WMO Secretary-General Michel Jarraud, in a prepared statement. "Rising concentrations of heat-trapping greenhouse gases are changing our climate, with far reaching implications for our environment and our oceans, which are absorbing both carbon dioxide and heat.”
For all of its extremes, however, the decade 2001-10 was also noteworthy for the decline in the number of people killed by extreme weather – despite the high tolls from hurricane Katrina, the 2003 heat wave in Europe and in Russia in 2010, and the floods that inundated Pakistan.
The number of people who succumbed to heat waves during the decade rose by 2,000 percent over the prior decade. But the number of fatalities from storms dropped 16 percent and the number killed from floods fell by 43 percent over the prior decade. Overall, the number of casualties from weather and climate extremes dropped by 20 percent between the 1991-2000 period and 2001-10.
The WMO, which released its decade in review Wednesday, attributes the decline "in good part" to improved early-warning systems and higher states of preparedness.
The report's observation that 2001-10 is the warmest decade on record isn't new. Researchers have noted this at least since 2011. Still, the decade is noteworthy, as 9 in 10 of those years are among the 10 warmest years in the instrument record.
Even La Niña, one-half of the see-saw El Niño-La Niña climate cycle that occurs in the tropical Pacific, felt the heat. The pattern, formally known as the El Niño-Southern Oscillation, influences seasonal atmospheric circulation patterns far beyond its tropical home.
Global average temperatures during La Niña years typically are cooler than El Niño years or even the so-called La Nada years, when neither sibling has the upper hand. The year 2008 saw the warmest La Niña on record up to that point, according to the WMO. That has since been eclipsed by the La Niñas of 2011 and '12, according to data kept by the National Oceanic and Atmospheric Administration in Silver Spring, Md.
Indeed, as if to underscore the influence of climate change on natural variability, each La Niña but one since the mid-1970s has been warmer than the previous La Niña event. This coincided with a marked increase in global average temperatures.
The 2001-10 decade also has seen a slight cooling trend that has continued. Some political skeptics of global warming have cited this as evidence that global warming is over. Climate scientists are puzzling over the trend as well.
Global temperature records compiled at centers in the US and Britain show a similar ramp-up in temperatures from 1910 to the late '40s or early '50s, only to see a slight cooling trend that continued into the mid-to-late '70s, before global average temperatures resumed their increase.
Researchers have been hunting for the "missing heat." Some suggest that some of the heat has been taken up through melting polar and glacial ice. Others have pointed to reflective aerosol particles that likely increased with the coal-fired economic explosions in China and India over the past decade.
Still others are looking to the deep ocean as a repository for the missing heat. A team of scientists in the US and Britain published an analysis in May of ocean-temperature data during the decade. They conclude that a significant amount of the missing heat has gone into the deep ocean, below 700 meters. The work appeared May 23 in the journal Geophysical Research Letters.
Whatever the reason for the slight throttling back of temperature increases, temperatures remained significantly above the 1961-'90 base period the WMO uses for its data. And warming has remained significant enough to load the dice for the severe weather and climate events recorded during the decade, researchers say.
Citing data from NOAA, the WMO notes that the 2001-10 decade was the most active for Atlantic tropical cyclones since 1855, although in other regions, tropical-cyclone activity was near or below normal.
Since 1901, only the '50s was a wetter decade globally – significantly so. Even so, 2010 was the wettest year on record, globally.
Over the decade, the proportion of countries reporting record maximum temperatures has grown, while the proportion of countries reporting record lows has declined – anchoring a trend that has been building since the 1960s.
One of the most dramatic signs of change has come in the Arctic, which has seen an erratic but long-term decline in extent and thickness of summer sea ice – a crucial component of the global and regional climate system. In 2007, the extent reached a record low, some 39 percent below the long-term average. That record was eclipsed last year. Greenland's ice sheet and key regions of Antarctica's continental ice sheet also have been losing mass. The decade's largest declines in mass were seen in 2007 and '08, the WMO says, adding to sea-level rise already under way because of ocean warming.
The WMO acknowledges that while it's not yet possible to tie a specific storm or even heat wave directly to global warming, climate scientists "increasingly conclude that many recent events would have occurred in a different way – or would not have occurred at all – in the absence of climate change."