Fall in San Francisco: Jim Renwick’s AGU report

Jim RenwickI took part in the 2015 AGU Fall Meeting, held at the vast Moscone Center in San Francisco, 14-18 December. As always, it was an absolute cornucopia of everything to do with the Geo/Earth Sciences, from exoplanets to the earth’s core to climate change and science policy, delivered by over 20,000 geoscientists. The Fall Meeting is always a blast, a real mind-expander.

This year, I was committed to chairing sessions first thing on Monday morning and then again on Friday. Monday’s session was “Evaluating Reanalysis: What Can We Learn about Past Weather and Climate?” with my sub-session having a focus on polar regions. The Thu/Fri session was “Precipitation over Mountainous Terrain: Observations, Understanding, Modeling, and Future Prospects”. In between, I soaked up as much as I could, wandering the halls to hear and see fascinating presentations on climate history, science communication, sea ice, and designing climate change musicals for primary school children. Here’s a few highlights, my personal “tip of the iceberg” from this year’s meeting.

We are starting to understand more about how Antarctic sea ice varies. There’s lots of evidence now that increases in the Southern Hemisphere westerly winds brought about by ozone depletion and greenhouse gas increase initially cool the Antarctic coast and foster increases in sea ice (the “fast response”). But after a while, the “slow response” kicks in, where warmer water is upwelled around Antarctica leading to sea ice loss. The big question is, how long is “a while”? Some studies suggest it may be only five or so years while other suggest a few decades. Given the observations to date, a few decades looks more likely, but either way, the upward trend we’ve seen in Antarctic sea ice extent looks set to head the other way, and soon. It has been a very interesting year for Antarctic sea ice in 2015, with the big El Niño restricting ice extent over the winter and ice melt proceeding faster than normal through the spring, a real change from the past decade or so.

Speaking of the oceans, there was a lively session on “the hiatus”, the period of relatively slow atmospheric warming from the late 1990s to the early 2010s. When the last IPCC report was written, there was a whole section devoted to this phenomenon, even though several authors thought it wasn’t worth discussing, just a bit of noise on the warming trend. That’s clearly the mainstream view now.

The hiatus, if it ever did exist, is definitely over now.

Atmospheric warming has picked up again in the last few years, helped along mightily this year by a big El Niño. Over the past 15 years or so, the oceans have been absorbing vast quantities of heat. In fact, half of the increase in ocean heat content over the past century has occurred since 1997! The hiatus, if it ever did exist, is definitely over now. This is worrying, if we are now heading back into a more El Niño-dominated period (as in the 1980s and 90s). A period of rapid warming such as we saw in the late 20th century would blow global temperatures right through the 1.5°C warming “guardrail”, given we are already at one degree of warming since pre-industrial times.

There was a lot of great paleoclimate science presented at AGU. Alan Mix from Oregon State University gave a wonderful Emiliani Lecture on tipping points in the Pleistocene (the period from about 2.5 million years ago to the start of the Holocene around 12,000 years ago) and what they might mean for us and the future. Climate model experiments suggest that some of the rapid changes seen in the past record, such as the onset of deglaciations at the end of an ice age, might be the result of random variations in the oceans, the ice sheets and the atmosphere, on top of the slow changes wrought by variations in earth’s orbit and axial tilt (“Milankovitch cycles”)

Amazing to think that decisions we take (or don’t) in the next decade could define the future of global sea levels and the world’s coastlines for millennia to come.

The Milankovitch cycles set the average pace of the ice age cycle, but the exact timing of big events might be down to chance. This is not good news for humanity. As Alan Mix said, “We are the tipping point” – the current spike in CO2 concentrations is a much bigger and faster perturbation in the climate system than anything seen over the past 2-3 million years. Add a bit of randomness on top, and we may be in for some very rapid changes this century and next. Mix also showed very clearly that 2°C of warming may well be committing the globe to more than 25m of sea level rise, over 10,000 years. Amazing to think that decisions we take (or don’t) in the next decade could define the future of global sea levels and the world’s coastlines for millennia to come.

I learnt a little about “black carbon” last week, a chemical that is still somewhat mysterious, a product of “bad” or incomplete combustion such as biomass burning and diesel combustion. It has around 20 million times the capacity to absorb energy than carbon dioxide does. As new AGU Fellow Tami Bond said “It’s a good thing it falls out of the atmosphere quickly!” From this presentation, and many others, I was very strongly reminded that the atmosphere is just a bunch of gasses spread over the surface of the earth. There is no real barrier between us and outer space, the air just gets thinner as you go up. We are lucky that the current mix and volume of air sustains life. It would be a good idea to keep it that way.

There’s always lots of great stuff on communication at AGU. For instance, I heard about ClimateFeedback.org, a new facility to allow climate scientists to provide reviewed comment on the quality and veracity of web posts about climate change. If you want to know how to make climate change funny, check out the work of Josh Willis at NASA’s Jet propulsion Laboratory (JPL). He’s hilarious! A really great initiative for communicating with children is “Frontiers for Young Minds”, an e-journal containing articles written by scientists and reviewed by children. As the presenter pointed out, no one in the 3rd grade knows about power spectra, so keep it simple! That’s not “dumbing down” but using language that a young non-expert will understand.

As well as hearing some disturbing factoids (e.g. over half of Donald Trump supporters don’t even “believe” that atmospheric CO2 is increasing) I also became aware of some hopeful and inspiring initiatives, such as ClimateFeedback.org (see above), and the Long Now Foundation that fosters long-term thinking and aims to help preserve human civilisation for the next 10,000 years. As HG Wells said, “Human history becomes more and more a race between education and catastrophe”.

Author: James Renwick

I am a climate researcher with a background in atmospheric dynamics and statistics. I was involved in writing the 4th and 5th Assessment Reports for the IPCC and am also on the joint scientific committee of the World Climate Research Programme (WCRP).

5 thoughts on “Fall in San Francisco: Jim Renwick’s AGU report”

  1. Very good article. No doubt we will now have a couple of years of record temperatures, then a cooler period for a few years. Then the climate sceptics can start their global warming slogan “there has been no warming since 1916” to bore us all to tears, and create as much doubt and delay as possible.

    1. Thanks Bob. Of course we don’t know the exact sequence, but if we’re heading into a period like the 80’s and 90’s, and if CO2 concentrations don’t start to go down (almost certain, even is emissions do), we can expect at least another 0.4C of warming by 2040-ish.

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