Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.
By: Dr. Jeff Masters , 17:22 GMT le 27 janvier 2012
The year 2011 tied with 1997 as the 11th warmest year since records began in 1880, NOAA's National Climatic Data Center said last week. NASA rated 2011 as the 9th warmest on record. Land temperatures were the 8th warmest on record, and ocean temperatures, the 11th warmest. For the Arctic, which has warmed about twice as much as the rest of the planet, 2011 was the warmest year on record (between 64°N and 90°N latitude.) The year 2011 was also the 2nd wettest year over land on record, as evidenced by some of the unprecedented flooding Earth witnessed. The wettest year over land was the previous year, 2010.
Figure 1. Departure of global temperature from average for 2011. The Arctic was the warmest region, relative to average. Image credit: NASA Earth Observatory.
How much of the warming in recent decades is due to natural causes?
The El Niño/La Niña cycle causes cyclical changes in global temperatures that average out to zero over the course of several decades. La Niña events bring a large amount of cold water to the surface in the equatorial Eastern Pacific, which cools global temperatures by up to 0.2°C. El Niño events have the opposite effect. The year 2011 was the warmest year on record when a La Niña event was present. Global temperatures were 0.12°C (0.2°F) cooler than the record warmest year for the planet (2010), and would very likely have been the warmest on record had an El Niño event been present instead.
Figure 2. Departure from average of annual global temperatures between 1950 - 2011, classified by phase of the El Niño-Southern Oscillation (ENSO). The year 2011 was the warmest year on record when a La Niña event was present. ENSO is a natural episodic fluctuation in sea surface temperature (El Niño/La Niña) and the air pressure of the overlying atmosphere (Southern Oscillation) across the equatorial Pacific Ocean. Over a period of months to a few years, ENSO fluctuates between warmer-than-average ocean surface waters (El Niño) and cooler-than-average ocean surface waters (La Niña) in that region. Image credit: National Climatic Data Center.
Correcting for natural causes to find the human contribution
We know that natural episodes of global warming or cooling in the distant past have been caused by changes in sunlight and volcanic dust. So, it is good to remove these natural causes of global temperature change over the past 33 years we have satellite data, to see what the human influence might have been during that time span. The three major surface temperature data sets (NCDC, GISS, and HadCRU) all show global temperatures have warmed by 0.16 - 0.17°C (0.28 - 0.30°F) per decade since satellite measurements began in 1979. The two satellite-based data sets of the lower atmosphere (UAH and RSS) give slightly less warming, about 0.14 - 0.15°C (.25 - .27°F) per decade (keep in mind that satellite measurements of the lower atmosphere temperature are affected much more strongly by volcanic eruptions and the El Niño phenomena than are surface-based measurements taken by weather stations.) A 2011 paper published by Grant Foster and Stefan Rahmstorf, Global temperature evolution 1979 - 2010, took the five major global temperature data sets and adjusted them to remove the influences of natural variations in sunlight, volcanic dust, and the El Niño/La Niña cycle. The researchers found that adjusting for these natural effects did not change the observed trend in global temperatures, which remained between 0.14 - 0.17°C (0.25 - 0.31°F) per decade in all five data sets. The warmest years since 1979 were 2010 and 2009 in all five adjusted data sets. Since the known natural causes of global warming have little to do with the observed increase in global temperatures over the past 33 years, either human activity or some unknown natural source is responsible for the global warming during that time period.
Figure 3. Departure from average of annual global temperatures between 1979 - 2010, adjusted to remove natural variations due to fluctuations in the El Niño-Southern Oscillation (ENSO) cycle, dust from volcanic eruptions, and changes in sunlight. The five most frequently-cited global temperature records are presented: surface temperature estimates by NASA's GISS, HadCRU from the UK, and NOAA's NCDC, and satellite-based lower-atmosphere estimates from Remote Sensing Systems, Inc. (RSS) and the University of Alabama Huntsville (UAH.) Image credit Global temperature evolution 1979- 2010 by Grant Foster and Stefan Rahmstorf, Environ. Res. Lett. 6, 2011, 044022 doi:10.1088/1748-9326/6/4/044022.
Commentary: what do climate scientists think?
Some scientists have proposed that previously unknown natural causes could be responsible for global warming, such as a decrease in cloud-producing galactic cosmic rays. Others have proposed that the climate may be responding to the heat-trapping effects of carbon dioxide by producing more clouds, which reflect away sunlight and offset the added heat-trapping gases. These theories have little support among actively publishing climate scientists. Despite public belief that climate scientists are divided about the human contribution to our changing climate, polling data show high agreement among climate scientists that humans are significantly affecting the climate. A 2008 poll of actively publishing climate scientists found that 97% said yes to the question, "Do you think human activity is a significant contributing factor in changing mean global temperatures?" In my personal experience interacting with climate scientists, I have found near-universal support for this position. For example, I am confident that all 23 climate scientists and meteorologists whom I am personally acquainted with at the University of Michigan's Department of Atmospheric, Oceanic, and Space Science would agree that "human activity is a significant contributing factor in changing mean global temperatures." It is good that we have scientists skeptical of the prevailing consensus challenging it, though, because that is how scientific progress is made. It may be that one of the scientists making these challenges will turn out to be the next Einstein or Galileo, and overthrow the conventional scientific wisdom on climate change. But Einsteins and Galileos don't come along very often. The history of science is littered with tens of thousands of discredited scientific papers that challenged the accepted scientific consensus and lost. If we rely on hopes that the next Einstein or Galileo will successfully overthrow the current scientific consensus on climate change, we are making a high-stakes, low-probability-of-success gamble on the future of civilization. The richest and most powerful corporations in world history, the oil companies, have spent hundreds of millions of dollars to push us to take this gamble, and their efforts have been very successful. Advertising works, particularly when your competition has little money to spend to oppose you.
Where is the climate headed?
The 2007 United Nations-sponsored IPCC report predicted that global temperatures between 2007 and 2030 should rise by an average of 0.2°C (0.36°F) per decade. The observed warming over the past 30 years is 15 - 30% below that (but within the range of uncertainty given by the 2007 IPCC climate models.) Most of the increase in global temperatures during the past 30 years occurred in the 1980s and 1990s. The 2000s have seen relatively flat temperatures, despite increasing CO2 emissions by humans. The lower-than-expected warming may be partially due to a sharp decrease in stratospheric water vapor that began after 2000. The missing heat may also be going into the deep ocean waters below about 1,000 feet (300 meters), as part of a decades-long cycle that will bring extra heat to the surface years from now. Regardless, the laws of physics demand that the huge amount of heat-trapping gases humans are pumping into the atmosphere must be significantly altering the weather and climate, even if we are seeing a lower than predicted warming. As wunderground's climate change blogger, Dr. Ricky Rood said in a recent post,Changing the Conversation: Extreme Weather and Climate: "Given that greenhouse gases are well-known to hold energy close to the Earth, those who deny a human-caused impact on weather need to pose a viable mechanism of how the Earth can hold in more energy and the weather not be changed. Think about it."
Our recent unusual weather has made me think about this a lot. The natural weather rhythms I've grown to used to during my 30 years as a meteorologist have become significantly disrupted over the past few years. Many of Earth's major atmospheric circulation patterns have seen significant shifts and unprecedented behavior; new patterns that were unknown have emerged, and extreme weather events were incredibly intense and numerous during 2010 - 2011. It boggles my mind that in 2011, the U.S. saw 14 - 17 billion-dollar weather disasters, three of which matched or exceeded some of the most iconic and destructive weather events in U.S. history--the "Super" tornado outbreak of 1974, the Dust Bowl summer of 1936, and the great Mississippi River flood of 1927. I appeared on PBS News Hour on December 28 (video here) to argue that watching the weather over the past two years has been like watching a famous baseball hitter on steroids--an analogy used in the past by climate scientists Tony Broccoli and Jerry Meehl. We're used to seeing the slugger hit the ball out of the park, but not with the frequency he's hitting them now that he's on steroids. Moreover, some of the home runs now land way back in the seats where no one has ever been able to hit a home run before. We can't say that any particular home run would not have occurred without the steroids, but the increase in home runs and the unprecedented ultra-long balls are highly suspicious. Similarly, Earth's 0.6°C (1°F) warming and 4% increase in global water vapor since 1970 have created an atmosphere on steroids. A warmer atmosphere has more energy to power stronger storms, hotter heat waves, more intense droughts, and heavier flooding rains. Natural weather patterns could have caused some of the extreme events we witnessed during 2010 - 2011, and these years likely would have been naturally extreme years even without climate change. But it strains the bounds of credulity that all of the extreme weather events--some of them 1-in-1000-year type events--could have occurred without a significant change to the base climate state. Mother Nature is now able to hit the ball out of the park more often, and with much more power, thanks to the extra energy global warming has put into the atmosphere.
Extreme weather years like 2010 and 2011 are very likely to increase in frequency, since there is a delay of several decades between when we put heat-trapping gases into the atmosphere and when the climate fully responds. This is because Earth's oceans take so long to heat up when extra heat is added to the atmosphere (think about how long it takes it takes for a lake to heat up during summer.) Due to this lag, we are just now experiencing the full effect of CO2 emitted by the late 1980s; since CO2 has been increasing by 1 - 3% per year since then, there is a lot more climate change "in the pipeline" we cannot avoid. We've set in motion a dangerous boulder of climate change that is rolling downhill, and it is too late to avoid major damage when it hits full-force several decades from now. However, we can reduce the ultimate severity of the damage with strong and rapid action. A boulder rolling downhill can be deflected in its path more readily early in its course, before it gains too much momentum in its downward rush. For example, the International Energy Agency estimates that every dollar we invest in alternative energy before 2020 will save $4.30 later. There are many talented and dedicated people working very hard to deflect the downhill-rolling boulder of climate change--but they need a lot more help very soon.
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