Tales From AGU: 2011 Edition
I attended the fall meeting of the American Geophysical Union last week—the world's largest gathering of earth scientists. This year there were 20,000 attendees, 12,000 poster presentations, 6,000 oral presentations, and 250 exhibitors, along with workshops, town halls, and social/networking events. It's always inspiring to be surrounded by so many brilliant minds, and I try to absorb as much as possible every year. This year was no exception: over 4 days I attended dozens of talks on climate change, hurricanes, cloud physics, and science communication. Those subjects don't even scratch the surface of all that the AGU Fall Meeting has to offer: ocean sciences, rock physics, bio-geoscience, seismology, and planetary science, to name just a few others. Here are a few of the talks I attended:
Evidence for an anthropogenic contribution to recent ocean-driven ice losses in West Antarctica
Dr. Eric Steig (Atmospheric Sciences, University of Washington)
Eric Steig (Earth and Space Sciences at U. Washington, as well as www.realclimate.org) gave a talk on Tuesday that presented evidence for a manmade contribution to ice loss in West Antarctica. The ice sheet and glaciers in West Antarctica, like the Pine Island glacier and the Thwaites glacier, have been melting rapidly into the Amundsen Sea—an arm of the Southern Ocean, which surrounds Antarctica. Scientists are very concerned about this because of the potential sea level rise that will result from melting so much ice into the ocean. It's well-established that this process is speeding up due to the intrusion of warm water underneath the glaciers' ice shelves, which melts the glacier at a critical point where land ends and ocean begins. The ocean floor in this region is situated in such a way that it is particularly susceptible to the relatively warm Circumpolar Deep Water (CDW) that flows around the continent of Antarctica.
Previous research has suggested that changes in the winds that blow around Antarctica have allowed or forced the warm deep water into the Amundsen Sea, speeding up the melting process. However, Steig argued that the cause is a little farther away than that, in the tropical Pacific, which, in one way, is linked to the Southern Ocean via the atmosphere. He showed that there is a significant link between sea surface temperature in the central tropical Pacific and the winds that flow over the region around the Amundsen Sea. Since we're pretty sure that manmade warming is the reason water is getting warmer in the central Pacific, it's likely that we can also link the manmade global warming to the melting in the West Antarctic.
A New Estimate of the Earth’s Land Surface Temperature History (Berkeley BEST)
Dr. Richard Muller (Physics, University of California at Berkeley) / Dr. Robert Rohde (Novim Group)
Berkeley's BEST temperature project was represented at the meeting by Dr. Robert Rohde, who is the lead scientist on the project under the founder and director, Dr. Richard Muller. Rohde was the lead author on the paper describing the project's averaging process.
This talk presented the results of the BEST study and the method, which I covered in a blog last month. But at the end of the presentation, Rohde showed this simple and visually pleasing animation of the BEST temperature reconstruction. Watch as temperature stations come online and the temperature anomalies (difference from average) increase with time:
Assessing hurricane vulnerability changes arising from climate variability and change
Dr. Greg Holland (NESL, NCAR)
Dr. Holland spoke about defining and quantifying hurricane risk, and considered whether this risk would change for some communities in a warming environment. A community that is susceptible to natural disasters, like hurricanes, needs to understand its vulnerability. Dr. Holland argued that vulnerability isn't the right word—they should consider "survivability." For example: which conveys the message you're trying to relay? "Are you vulnerable to this hurricane?" or "Can you survive this hurricane?" The next step is to define the hazard. In the case of hurricanes, the hazard is wind, waves, surge, flood, etc. Most people say hurricane intensity is the main cause of the hazard, when in reality this is not the case. How large the hurricane is (its radius, for example) and how fast the hurricane is moving (its translational speed) are actually more valuable indicators of hazard than intensity. Larger hurricanes tend to have bigger storm surges and produce more rain. Hurricanes that move faster tend to do less damage than the ones that are slow and "linger" over an area. Most people in the hurricane world have understood this for a while, but the question Holland raised in his talk is "how will these things change in the future, and how does that change the risk?" If we're assuming there will be a 5mph increase in hurricane intensity, on average, how does that change the risk? According to Holland, it doesn't, because intensity is not where the risk lies. He argues that if, in a warming world, hurricanes shrink in size and move faster, the risk is actually reduced. However, the question of what will actually happen to hurricanes in a warming world is still up for debate.
Hurricanes in a Warming Climate
Dr. Kerry Emanuel (Earth, Atmospheric, and Planetary Science, MIT)
Dr. Kerry Emanuel gave a good talk on tropical cyclones in a warming climate. We know that power dissipation (the potential destructiveness of a hurricane) and sea surface temperature are well-linked. Power dissipation has doubled in the last 30 years, and it's also linked to northern hemisphere air temperature on a decadal time scale. Potential intensity, or the maximum potential wind speed a hurricane can achieve given its environment, has also increased in the last 30 years, by 10% at least. Dr. Emanuel did an interesting analysis using the potential intensity theory where he shows that if Katrina had happened in 1980, it would have peaked at a category 4, not a category 5. An interesting question: would that have been enough to prevent the levees from breaking? Outflow temperature of hurricanes, the "exhaust" of the hurricane at the upper levels of the atmosphere, has cooled in the last 30 years, as well. In order to strengthen, a hurricane needs a cold upper atmosphere just as much as it needs a warm lower atmosphere. These "cold upper atmosphere" trends have not been forecasted in the global climate models, so arguably, they cannot forecast a future trend in hurricane strength.
Hurricane Katrina on August 28, 2005. Source: NOAA
Evaluating tropical cyclone genesis in four global models
Daniel Halperin (Earth, Ocean, and Atmospheric Science, Florida State University)
Daniel Halperin from Florida State University presented an evaluation of tropical cyclone genesis in four models from 2004 to 2010: the CMC, GFS, UKMET and NOGAPS. Unfortunately he did not include the ECMWF in the study, though we know that it would have out-performed the four included here. He used common evaluation metrics: hits and false alarms. The idea here is that you want the model to have a low false alarm rate (forecasting a hurricane when it's not going to happen) and a high hit rate (correctly forecasting a hurricane).
The Canadian CMC has improved since 2004, but still has a ~55% false alarm rate as of 2010. Its worst year was 2007, when nearly 80% of its forecasts were false alarms.
The GFS has improved markedly, especially since 2007 when it had almost a 90% false alarm rate.
In 2010, that was down to ~38%, and their "correctly predicted" rate is up to almost 50% from 10%. However, spatially, Halperin shows that the GFS misses almost all tropical cyclones that form in the Gulf of Mexico. As is expected, it performs best in the central Atlantic, where tropical cyclones develop from waves that track across the ocean from Africa and are relatively easy to pick up on.
NOGAPS false alarms are down to around 50% from nearly 100% in 2004-2006, but their rate of correct forecasts is still pretty low, just 12% in 2010.
The UKMET has been consistent since 2004, with false alarms hovering around 50%, and correct forecasts steadily increasing from 20% in 2004 to 45% in 2010. However, like the GFS, the UKMET misses almost all tropical cyclones that form in the Gulf of Mexico.
In summary, overall, the GFS and the UKMET are performing best out of the four models tested in this study.
Another big theme at this year's conference was science communication. I'll be back some time this week with a post on that, which will include the 2011 update to the "six Americas."