SAN FRANCISCO, CA., DEC. 10, 2013 — Judith Lean gave a great lecture here this morning that clearly and succinctly laid out the last decade’s changes in climate, and it gave some firm thinking on why surface temperatures have been flat (“the hiatus“).

Her message was simple: climate change is a mixture of many factors, and not just increases in carbon dioxide.

After a few slides on the great amount of confusion this causes the public and politicians, Lean simply presented (and presented simply) the relevant factors that affect surface temperature trends, showing time series for all of them: greenhouse gases, volcanic aerosols, El Niño’s and La Niña’s, the solar constant (a misnomer, because it varies over the course of a solar cycle and over the course of decades and centuries, but it’s the word everyone uses), and natural variability.

Going through all the data, Lean said that in the last several years surface temperatures have been depressed by a tendency toward a La Niña state in the Pacific Ocean, some decrease in the solar constant, and some volcanic aerosols.

She used a “statistical model” to fit all these together — basically fitting past data to factors that represent each of the possible forcings — and found that the most deviation from expectations since 2008 has occurred during the northern hemisphere winter and over northern hemisphere land masses, and that they often occur in winters that follow a large melt in the Arctic the previous summer (such as the large melts of 2007 and 2012).

Lean speculated that melting Arctic sea ice is acting as a negative climate feedback, changing northern mid-latitude weather patterns for months after a large melt.

There was more, but this was one of the major takeaways. I hope to get a copy of her presentation soon, and will post a few graphs from it (see followup posting).

Her lecture was sparkling — simple and clear, but drilling right into the relevant factors. She made it clear that there just isn’t any reason to doubt greenhouse warming for carbon dioxide, etc. — too much else goes on with short-term climate that can easily produce the kind of hiatus we’ve seen in the last 10 to 15 years.

In fact, expect more of them, she said.

Topics: Climate Science