September 21, 2005

Polar Ice Cap Mystery

Okay, I'll be the first to admit, I don't know a whole heck of a lot about the science behind climate change. But luckily, I did stay at a Holiday Inn… No, even better, I did read Powerline this morning, which tells me this:
While it is theoretically possible for human and animal activities to affect the climate on Earth, the main factor causing fluctuations in temperatures on this planet, as on Mars, is variability in energy output from the Sun.
Uh... what? Powerline's little foray into "science" was spurred by recent news that polar ice-caps on Mars seem to be melting. Figuring that maybe, just maybe, there were better sources out there than Powerline, I googled the topic and got a handy Q&A page from the National Oceanic and Atmospheric Admistration which addresses this very question. Key stuff in bold:
Q: Can the observed changes be explained by natural variability, including changes in solar output?

Since our entire climate system is fundamentally driven by energy from the sun, it stands to reason that if the sun's energy output were to change, then so would the climate. Since the advent of space-borne measurements in the late 1970s, solar output has indeed been shown to vary. There appears to be confirmation of earlier suggestions of an 11 (and 22) year cycle of irradiance. With only 20 years of reliable measurements however, it is difficult to deduce a trend. But, from the short record we have so far, the trend in solar irradiance is estimated at ~0.09 W/m2 compared to 0.4 W/m2 from well-mixed greenhouse gases. There are many indications that the sun also has a longer-term variation which has potentially contributed to the century-scale forcing to a greater degree. There is though, a great deal of uncertainty in estimates of solar irradiance beyond what can be measured by satellites, and still the contribution of direct solar irradiance forcing is small compared to the greenhouse gas component. However, our understanding of the indirect effects of changes in solar output and feedbacks in the climate system is minimal. There is much need to refine our understanding of key natural forcing mechanisms of the climate, including solar irradiance changes, in order to reduce uncertainty in our projections of future climate change.

In addition to changes in energy from the sun itself, the Earth's position and orientation relative to the sun (our orbit) also varies slightly, thereby bringing us closer and further away from the sun in predictable cycles (called Milankovitch cycles). Variations in these cycles are believed to be the cause of Earth's ice-ages (glacials). Particularly important for the development of glacials is the radiation receipt at high northern latitudes. Diminishing radiation at these latitudes during the summer months would have enabled winter snow and ice cover to persist throughout the year, eventually leading to a permanent snow- or icepack. While Milankovitch cycles have tremendous value as a theory to explain ice-ages and long-term changes in the climate, they are unlikely to have very much impact on the decade-century timescale. Over several centuries, it may be possible to observe the effect of these orbital parameters, however for the prediction of climate change in the 21st century, these changes will be far less important than radiative forcing from greenhouse gases.
Good to know, I think I'll stick with that. Nevertheless, what does explain the melting ice-caps in Mars? Clearly not a human-induced increase in greenhouse gases—so what then? Changes in Mars' position relative to the sun? Or is Powerline right and solar variability is far stronger than, you know, most scientists believe (this, naturally, is the most implausible theory of the bunch)? To my (very) untrained eye, it seems like an actual mystery, although I guess this all depends on how quickly the Mars ice caps are melting—which isn't really clarified anywhere.

MORE: I would hope it was clear that I'm not trying to deny the fact of human-induced global warming here on Earth, but maybe it wasn't. Nevertheless, here's more on climate change on Mars.
-- Brad Plumer 1:24 PM || ||