Friday, March 26, 2010

Particle Physics, Climate Change, and Dinner With Vera Rubin, Part I: Interview with A Climate Scientist

Finally, my APS April Meeting article is done!

Read article full text on the SPS website.  

By far the biggest event in the avenue of energy and environment was the plenary talk by Naval Research Observatory scientist Dr. Judith Lean, entitled "Surface Temperature Responses to Natural and Anthropogenic Influences: Past, Present, and Future." In light of such recent events as the email hacking at the Climate Research Unit in East Anglia, and the APS's ongoing discussion on its official climate position, Dr. Lean's talk was destined to be a hot topic, and was well attended. In the talk, Dr. Lean presented climate data from the past 30 years, revealing the contributions of both natural and anthropogenic factors to global temperature changes.

"Surface temperature is the equilibrium of incoming and outgoing radiation, modulated by several processes," explained Dr. Lean. Although human additions to concentrations of so-called greenhouse gases, such as carbon dioxide and chlorofluorocarbons, have received much attention of late, many other natural processes have observable effects on global temperature. The presence or absence of an El Niño Southern Oscillation can create warming in equatorial oceans, aerosols from volcanic eruptions can block incoming sunlight and create periods of cooling, and the sun itself goes through cycles of increasing and decreasing irradiance. When these decoupled effects are looked at together, they explain some of the temperature patterns of the last 30 years, yet anthropogenic greenhouse gases are so far the only explanation for the background rise of global temperatures in the past two decades.

In other words, it does not take a steady increase in temperatures across the globe to indicate that humans have altered global temperatures. Rather, what we see is an upward trend that is modulated by natural processes. According to Dr. Lean, we can expect that upward trend to dominate in the long run.

A common critique of anthropogenic warming asserts that the global temperature changes can instead be accredited to the changing brightness of the sun. Dr. Lean admits that solar irradiance has been something of a wild card: direct measurement of irradiance only goes back to 1978, and even the IPCC models do not deal with variances in irradiance, because of the difficulty in modeling the changes in oceanic and meteorological processes that can be caused by a change in solar irradiance. Yet Dr. Lean's work for the NRO includes modeling the changing magnetic forces within the sun, in hopes of yielding a more long-term picture of the patterns of solar irradiance, so she is well qualified to speak to the variation in solar irradiance itself. For solar variation to account for the nearly 0.9 Kelvin change that has been observed over the past 150 years, says Dr. Lean, “you'd have to say that the sun has varied five times more than we think it has varied, and you'd have to say the earth is really sensitive to sun's variations but at the same time is insensitive to increasing greenhouse gasses." To that she adds that taking the anthropogenic and natural processes together gives a consistent picture of both the last 30 years and the last 150 years, something solar variation does not do.

Immediately after her presentation she was swamped with enthusiastic crowd members, eager to ask questions or obtain a copy of her slides. Those slides contained simple and informative graphs of the past three decades of climate data, information that Dr. Lean admitted to me that she and her colleague almost didn't bother publishing. "We really focused on the forecasting at first, because we didn't think there was enough science in just explaining the past 30 years," she told me. "We thought, 'everyone knows that', you know, here's an ENSO [El Niño], here's a volcano. But it turns out that nobody knows that!" When working to determine how much models can or cannot tell us about the future, we sometimes forget that there is much that direct observation from the past can say.

Dr. Lean and I discussed the importance of communicating climate science effectively. I asked her, as a climate scientist, what she wanted the public to know about climate change, and she reiterated the main points she'd made in her presentation. "The climate varies for lots of reasons, on different timescales, by different amounts, and due to different things," she reminds. "There's this expectation that if anthropogenic gasses are causing climate change, then as proof of that we'll see global temperatures going monotonically up and up. But just because you see ups and downs, doesn't mean that anthropogenic gasses aren't affecting the climate."

She adds that, "The sun actually does appear to have a role, but it's not a very dominant role."

I asked Dr. Lean how she would improve the current climate science study, if she had more resources. She laughed, and said she would put in place the best observing system that money could buy.

Stay tuned for excerpts from Part II and III over the next few days!

No comments:

Post a Comment