Monday, June 29, 2009

Solar Cycles and Global Climate

Ryan previously made reference to an idea I get asked about a lot, so I thought I'd leave a quick post with my two cents on the matter. While I am a solar physicist, I do not work on issues of solar irradiance or even solar changes to space weather, so I am not an expert in this exact field. However, I do work on dynamo models that explore the sun's magnetic behavior over hundreds of year. Basically, I am not the best person to ask about this, but I'm familiar with what is going on in the field.

So here's the question: is the sun causing global warming? And here's the quick answer: no, at least not in the past 50 years. Here's a more detailed response:

The sun's magnetic activity does appear to impact the Earth's climate. Historical records indicate that a lull in solar activity called the Maunder minimum corresponded to a very cold period (especially in Europe) from 1645 to 1715 AD. The exact mechanism linking solar activity and climate is not well understood because the variations in total solar luminosity are extremely small (less than 0.1%), which rules out direct effects. However, there are several indirect effects that may drive climate changes including modifications to upper-atmospheric chemistry and increased cosmic ray fluxes changing cloud formation reates. Whatever the mechanism, the sun appears to drive changes of about +/- 0.5 degrees C. But that cannot account for the rapid change in temperatures in the last 50 years. And here's a figure that shows it:
This figure, compiled by the good people at NASA-Marshall's Solar Physics Group shows some correlation between solar activity and temperature, but the large spike in temperature since 1950 does not have a corresponding increase in solar activity. So while the sun might cause something on the order to a 0.5 degree change in global average temperature, it does not explain the current warming trend. Atmospheric CO2 concentration does a far better job matching the warming trend. So while the sun is a player in our climate, it is not the dominate agent of change right now.

As a side note, these solar forcing effects are just one more set of parameters that get fed into climate models, providing more poorly constrained parameters to fiddle with.

1 comment:

  1. Lies! Lies! All of it! I refuse to believe any of it!

    On a serious note. Two things: It will be interesting to see if the trend of global temperatures will continue to follow the rise in CO2.

    The second thing is, how much of the CO2 comes from humans? Volcanoes can put out a lot of CO2, along with natural seepage from the ground. I have never seen anyone present (or even try to present) and estimate of how much of the atmospheric CO2 comes from natural sources and how much comes from us. Given the fairly constant rise of CO2 that would indicate that it may not come from volcanoes as they are one time brief events and thus the rise should have small spikes in it to correspond to eruptions. But that does not mean that they do not contribute or that other more constant sources don't contribute.


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