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NOAA’s National Climatic Data Center recently announced that this past winter was the coolest since 2001, for the U.S. and the globe. Some people say that recent cooler temperatures mean that global warming has stopped. I've said it before, and I'll say it again: A single season can’t determine a long-term trend any more than a single month can.
However, the recent "cooler temperatures" offer a great opportunity to talk about short-term, natural climate variation. The long-term trend is clear – temperatures are going up because of increasing levels of greenhouse gases – but there will always be bumps and wiggles along the way. This past winter is one of those wiggles. What caused it, and how well do scientists understand these sorts of short-term changes?
Short-term climate variation is normal, and scientists understand quite a bit of it. One of the biggest causes of short-term internal climate variation is the El Niño Southern Oscillation (ENSO) cycle. (Note: On Wednesday there was a diary about a deceptive climate change graph. It mentioned La Niña’s effect on recent temperatures, which I also talk about here. I hope you'll pardon some necessary overlap.)
I'll go over the very basics of ENSO here, but I encourage you to head over to RealClimate's extremely thorough post about ENSO. Among other things, it describes the interesting history of how the two components of ENSO – the oceanic ("El Niño") and atmospheric ("Southern Oscillation") – were described separately, decades before scientists realized there was a connection between the two.
To start, check out this animation of the oceanic component of an idealized ENSO cycle. You're essentially looking northwest across the Pacific Ocean. The "EQ" label to the right shows the equator on the western edge of South America. The colors show sea surface temperature and the surface undulates up and down to demonstrate changes in sea level.
The animation starts during El Niño, when surface waters in the eastern equatorial Pacific are unusually warm. All that hot water transfers heat to the atmosphere, so global temperatures tend to be high in El Niño years. The animation then transitions to La Niña, which is essentially the opposite of El Niño. Winds and ocean currents bring cold, deep water up to the surface of the eastern equatorial Pacific Ocean, and temperatures tend to be cooler worldwide.
La Niña conditions began developing last year (yet 2007 was still one of the warmest years on record!). Meteorologists say this winter’s La Niña is now the strongest (coldest) one since 1988/89. And La Niña isn't the only factor in play. For example, total solar irradiance is currently at the minimum in the Sun's normal cycle. As a result, it’s no surprise that temperatures this winter have been cooler than in recent years. Even so, cooling this winter didn’t even come close to offsetting warming over the past 50 years.
Based on data from NASA GISS ("Seasonal Mean Temperature Change" near the bottom of this page). Temperatures are relative to the 1951-1980 average; each dot is a 3-month season.
Scientists can often explain the temperature record’s bumps and wiggles during or after the fact. But what about in advance – can climate models project short-term climate variation? Until very recently, the answer was no. That’s why it was so exciting when, last summer, researchers described a new and improved climate model that predicts internal climate variability.
When the scientists did a 10-year simulation (2005-2015), their model predicted that natural internal variability would offset human-caused global warming until some time this year, after which significant warming would resume. In fact, the model predicted that at least half the years between 2009 and 2015 would exceed the warmest year currently on record.
Time will tell how good that prediction is, and certainly this and other models will continue to improve. But the general result makes perfect sense. Consider what we know regarding this winter’s cooler temperatures: La Niña conditions typically last only a year or two. In contrast, greenhouse gases can warm the atmosphere for centuries. That’s why scientists expect that, unless we start making big cuts in emissions, the overall long-term trend will continue to be warming.
Further Reading
- The always excellent RealClimate blog has
a whole category of posts dedicated to ENSO, but
this one is my favorite.
- NOAA's
La Niña page
- NASA's
analysis of 2007 temperatures
- NASA's
Solar Dynamics Observatory
- You can find more posts on climate change science, policy, and news on
Climate 411