James Hansen has published a short piece, Scientific reticence and sea level rise [PDF], which I highly recommend. The abstract and some selected clips:

I suggest that a ‘scientific reticence’ is inhibiting the communication of a threat of a potentially large sea level rise. Delay is dangerous because of system inertias that could create a situation with future sea level changes out of our control. I argue for calling together a panel of scientific leaders to hear evidence and issue a prompt plain-written report on current understanding of the sea level change issue...

I believe there is a pressure on scientists to be conservative. Papers are accepted for publication more readily if they do not push too far and are larded with caveats. Caveats are essential to science, being born in skepticism, which is essential to the process of investigation and verification. But there is a question of degree. A tendency for ‘gradualism’ as new evidence comes to light may be ill-suited for communication, when an issue with a short time fuse is concerned...

The most compelling data for the net change of ice sheets is provided by the gravity satellite mission GRACE, which shows that both Greenland (Chen et al 2006) and Antarctica (Velicogna andWahr 2006) are losingmass at substantial rates. The most recent analyses of the satellite data (Klosco) confirm that Greenland and Antarctica are each losing mass at a rate of about 150 cubic kilometers per year, with the Antarctic mass loss primarily in West Antarctica. These rates of mass loss are at least a doubling of rates of several years earlier, and only a decade earlier these ice sheets were much closer to mass balance (Cazenave 2006).

The Antarctic data are the most disconcerting. Warming there has been limited in recent decades, at least in part due to the effects of ozone depletion (Shindell and Schmidt 2004). The fact thatWest Antarctica is losing mass at a significant rate suggests that the thinning ice shelves are already beginning to have an effect on ice discharge rates. Warming of the ocean surface around Antarctica (Hansen et al 2006a)is small compared with the rest of world, consistent with climate model simulations (IPCC 2007), but that limited warming is expected to increase (Hansen et al 2006b). The detection of recent, increasing summer surface melt on West Antarctica (Nghiem et al 2007) raises the danger that feedbacks among these processes could lead to nonlinear growth of ice discharge from Antarctica...

The nonlinearity of the ice sheet problem makes it impossible to accurately predict the sea level change on a specific date. However, as a physicist, I find it almost inconceivable that BAU climate change would not yield a sea level change of the order of meters on the century timescale. The threat of a large sea level change is a principal element in our argument (Hansen et al 2006a, 2006b, 2007) that the global community must aim to keep additional global warming less than 1°C above the 2000 temperature, and even 1°C may be too great. In turn, this implies a CO2 limit of about 450 ppm, or less. Such scenarios are dramatically different than BAU, requiring almost immediate changes to get on a fundamentally different energy and greenhouse gas emissions path..

Important decisions are being made now and in the near future. An example is the large number of new efforts to make liquid fuels from coal, and a resurgence of plans for energy-intensive ‘cooking’ of tar-shale mountains to squeeze out liquid hydrocarbon fuels. These are just the sort of actions needed to preserve a BAU greenhouse gas path indefinitely. We know enough about the carbon cycle to say that at least of the order of a quarter of the CO2 emitted in burning fossil fuels under a BAU scenario will stay in the air for an eternity, the latter defined practically as more than 500 years. Readily available conventional oil and gas are enough to take atmospheric CO2 to a level of the order of 450 ppm.

In this circumstance it seems vital that we provide the best information we can about the threat to the great ice sheets posed by human-made climate change. This information, and appropriate caveats, should be provided publicly, and in plain language. The best suggestion I can think of is for the National Academy of Sciences to carry out a study, in the tradition of the Charney and Cicerone reports on global warming. I would be glad to hear alternative suggestions.

Please take the few minutes necessary to download and read the whole thing. As you can tell from the sections I quoted, this is a rare example of “hard climate science” and “soft human factors” in the same paper, and it’s written by the world’s most famous climate scientist.

This reticence, to use Hansen’s word, is something I’ve been thinking about a great deal in recent weeks, partially due to discussions in a couple of e-mail lists. In the interest of not being a reticent blogger, let me say that I find this situation endlessly frustrating. I’ve spoken with several people about this phenomenon and learned that many who study climate, whether they are actual climate scientists or not, hold much more dire views about the science than they say publicly. Note that this is not a case of pessimism over the probability that we’ll take effective mitigation steps, leading to a conclusion that a BAU path will have horrible consequences. It’s a judgment about the science itself, as in the projections of the effect (such as sea level rise) associated with any given emissions path.

This leaves experts, and by extension, the rest of us, hanging in the balance as scientists decide whether to say what they believe is true vs. what they can prove is true. It’s a contest of heart vs. head, eerily reminiscent of the scene in countless TV shows and movies where a district attorney argues with police and says they’re right and the suspect in question is surely guilty, but he or she can’t file charges without enough evidence to prove it in court. Sadly, those story lines all too often result in the suspect committing another crime, one that could have been prevented with a more creative or assertive DA.[1] We’re now in a position of hoping that the instincts of many climate experts are wrong, not about “who done it” (it was us, with CO2, everywhere), but how bad it will or could be. When we’re talking about large numbers of such highly trained people, that’s not a comforting situation.

I want science to be able to be reticent, to make continual progress through small forward steps and very few backward ones. The timing of our current situation makes that approach, science’s own BAU path, as it were, very risky. We need scientists willing to stand up, pound the table, and tell the world in a forceful but scientifically responsible way that this situation is far more urgent than politicians and lay people recognize and more urgent than most scientists have said publicly until now. Sadly, finding such scientists is about as difficult as finding lay people who will listen to them or politicians willing to sacrifice short-term opportunism for gains that won’t be realized until long after they’re out of office.

[1] Please don’t read too much into this example and my views on law enforcement. If all you use is this one example taken from entertainment you’re likely to be conspicuously wrong.


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