‘Solar variability does not explain late-20th-century warming’, says the title of a short paper published earlier this year by Philip Duffy, Ben Santer and Tom Wigley in Physics Today. The reason I bring up the topic of the sun and climate now is that an Australian Senator, Stephen Fielding of the Family First party, has recently been concerned that the solar variability could be a cause of recent warming, as the vote for the Carbon Pollution Reduction Scheme comes before the Upper House. Apparently, he got this information from the American Heartland Institute. Well, let me put the good Senator’s concerns to rest.
This topic was dealt with in some detail on BraveNewClimate last year, in the post ‘What if the sun got stuck?‘. There is also an excellent coverage of this issue here, here and here. As Graeme Pearman said in the ABC story linked above, it’s an old debate. Pearman:
“Senator Fielding might have just learnt about it, but in fact the science community has been aware of it for many years. The changes of output of the sun are well and truly documented. We’ve been observing this for over a hundred years. We understand that there was probably some warming earlier last century, due to changes of emissions from the sun, but no evidence that the recent warming is due to that. And therefore there’s no anticipation that that will be a major factor through this century.”
The Duffy et al. 2009 paper (download the PDF here) was written in response to an Opinion Piece published in Physics Today in March 2008, by Nicola Scafetta and Bruce West, entitled: “Is climate sensitive to solar variability?” (download here). I strongly recommend that you read the Duffy et al. paper in full (it’s only 2 information-packed pages long), but the conclusion really does say it all:
In summary, the hypothesis of Scafetta and West — that solar variability is the dominant climate influence during the late 20th century — is a non-solution to a non-problem. There is no problem because the history of global temperatures during the 20th century is adequately explained by known phenomena: greenhouse gases, volcanic eruptions, aerosols, and, yes, to a small degree, solar variability. That conventional explanation is simple, self-consistent, and relies on well established physics. The Scafetta and West hypothesis is a non-solution because it is inconsistent with a range of observations and invokes new an unproven physics. Extraordinary claims require extraordinary proof; Scafetta and West have failed to provide it.
It’s always been amazing to me that some people go to such lengths to try to explain most of the warming over the last 150 years by reference to the sun, rather than ascribing it to an increase in greenhouse gases (GHG). Both, obviously, can change the climate; no argument there. But what about the principle of parsimony, folks? This argument distorts it to the extreme.
The sun delivers our planet almost all of its energy, and an increase in total solar irradiance (for whatever reason) of just a few percent would have a profound effect on Earth’s temperatures. Likewise, a large increase in the concentration of carbon dioxide and other GHG (methane, nitrous oxide, CFCs etc.) would, on the basis of fundamental physics, be expected to invoke a serious planetary warming event.
So, let’s look at the problem in the simplest possible terms:
1. The Earth’s temperature is rising.
2. Solar activity and GHG both force the climate system.
3. There is no trend in solar activity.
4. There is an upwards trend in greenhouse gas concentration.
Simple reasoning will point to the trending driver (GHG) over the non-trending driver (solar) being the culprit. For a solar explanation to work, you not only have to explain why a climate forcing agent would be exerting a directional effect of the climate system when it itself is NOT changing — you also have to explain how that stationary agent is also able to negate another climate forcing agent that IS changing.