Won't Temperatures Everywhere Be Warmer?

Kansas City has enjoyed a very temperate fall this year - three 80 degree days in October!   While we used to call this stroke of luck an Indian Summer, now, everywhere I go, I hear people saying we're reaping the rewards of global warming. So it did not surprise me when I received this question from one of my students:

"I just have an off the wall question and I have always wondered this but have not found an answer just yet. To my understanding, an indication of global warming is abnormal temperatures within seasons...i.e., 70 degree weather in November :) Global warming is supposed to be a "new age problem" and when I say new, I don't mean last year, but within the past 30-40 years. Well, how is it when I look at the news, it will say "Record High 87 in November 1917" LOL? Has global warming always existed? I think I read it somewhere but if that is the case why are people making it seem as if people that drive cars "created" this problem?"

I don't think her question is off the wall at all, and from what I can tell, I think her confusion is pretty widely shared.  I think the term "global warming" implies certain things, like warmer temperatures everywhere, or as this student misunderstood, unusually seasonable weather.  So, knowing that when one student asks, many have the question, I thought I'd clear this up a little bit for anyone else who's wondering.

The answer to this question is multi-layered, something that no doubt adds to confusion.  Weather patterns are effected by local events, meaning weather events that arise extemporaneously from local conditions, and also more generally by changes in global weather patterns.  El Nino, for example, was a blip - a local phenomenon, while temperature trending is considered to be a result of global warming.

To further confuse the matter, "Global warming" is one of those terms with a scientific meaning different than everyday language use might suggest.  While temperatures impacted by global warming are predicted to be warmer in some locales, they are simultaneously predicted to become significantly cooler in other locations. The warming part has to do with what happens, particularly at the poles, as greenhouse gases trap the sun's heat (that would normally "bounce" or be reflected off the earth's surface but instead are caught within the gaseous layer), causing ice caps to melt into the sea and other atmospheric changes.  The influx of cold water, in turn, cause a domino effect of change in temperature and density of ocean currents, and changing weather patterns.  So the greenhouse gas effect is just a trigger that starts a weather change ball rolling. Other factors have to be explored to understand the end result. Probably the biggest weather impacts will be the result of deep ocean current changes, which I'll explain in a minute.

Interestingly, the impacts of these trapped gases will not result in uniformly warmer climates. Using the term "climate change" would be somewhat more accurate, because the wind and the ocean's currents will move heat around the globe in ways that change snowfall and rain patterns, as well as heat and cooling patterns.

As for "hottest year" or "record high" data, there have always been cycles and "events." The ash from a rupturing volcano can change the atmospheric conditions in ways that temporarily cool.   El Nino caused a record high temperature in 1998 (some say a hotter temp occurred in 2005).  Those who don't agree with global warming theories point to that peak temperature (which has been followed by lower temps) as proof that global warming doesn't exist. Those who do agree with global warming theories just say El Nino, overlayed on top of warming patterns, created the blip on the temperature map, but the overall pattern is still trending up.

However, despite the fact that temperatures are trending up, there will be some places on earth that experience cooler temperatures over the long-run.  This is different than extemporaneous temperature events. A longer term cooling trend will be localized to certain areas of the globe, a result of changes to the movement of ocean currents - both surface currents driven by wind, and deep subsurface currents that move as a result of temperature/saline density changes - which distributes heat around the world.  Changes in heat distribution patterns will result in both warmer and cooler patterns, depending on the location.

One of the main factors impacting our weather is something called the "Ocean Conveyor," the system of surface and deep currents that moves heat from the tropics to the northern waters, where heat releases, moderating northern weather.  Rather than try to describe this myself, let me quote a pretty simple description from Woods Hole:

"The equatorial sun warms the ocean surface and enhances evaporation in the tropics. This leaves the tropical ocean saltier. The Gulf Stream, a limb of the Ocean Conveyor [what scientists call the current patterns that circulates heated water around the globe], carries an enormous volume of heat-laden, salty water up the East Coast of the United States, and then northeast toward Europe.


This oceanic heat pump is an important mechanism for reducing equator-to-pole temperature differences. It moderates Earth’s climate, particularly in the North Atlantic region. Conveyor circulation increases the northward transport of warmer waters in the Gulf Stream by about 50 percent. At colder northern latitudes, the ocean releases this heat to the atmosphere—especially in winter when the atmosphere is colder than the ocean and ocean-atmosphere temperature gradients increase. The Conveyor warms North Atlantic regions by as much as 5° Celsius and significantly tempers average winter temperatures.


But records of past climates—from a variety of sources such as deep-sea sediments and ice-sheet cores—show that the Conveyor has slowed and shut down several times in the past. This shutdown curtailed heat delivery to the North Atlantic and caused substantial cooling throughout the region. One earth scientist has called the Conveyor “the Achilles’ heel of our climate system.”  http://www.whoi.edu/page.do?pid=12455&tid=282&cid=996

Global warming effects are predicted to result in significant change to water temperature/saline constituency, enough to bring about yet another change in these historic current movement patterns and the weather accompanying the currents. The results would impact both land habitat, and ocean ecosystems.  Slowing the movement of the conveyor system can mean colder winters in the northeast U.S. and Western Europe. But that is a result of the heat gains from greenhouse gas concentrations, not a negation of global warming.  Click this sentence to see Woods Hole's great set of frequently asked questions on frequent misconceptions about climate change.  It goes into much more detail than I have.


I hope this clears up some of the crazy inconsistencies between the words "global warming" and some of the stuff we experience. Here are some links, if you want to read more:

http://environment.nationalgeographic.com/environment/global-warming/gw-overview.html

http://www.pik-potsdam.de/~stefan/thc_fact_sheet.html

http://www.whoi.edu/page.do?pid=12457

http://www.wunderground.com/education/abruptclimate.asp