Does National Climate Assessment lack necessary nuance?

climate-change

The new, 800-plus page National Climate Assessment released today is “extraordinary” says John Holdren, the White House Science Advisor, and “the loudest alarm bell to date”  on the need for climate change action.

The report provides solid evidence that the climate is changing due to human activity and poses important risks, from excessive heat, to coastal inundation, to declines in soil moisture. A common thread throughout the report is that climate change is a people story, affecting Americans right now.

“Climate change, once considered an issue for a distant future, has moved firmly into the present,” the “highlights” report begins.

These take home messages on the immediacy of the issue as well as potential future risks are supported by data and current science. Yet, one could argue the report – in a few places – goes too far in attempting to stay-on message by glossing over some of the thornier issues in climate science and not sufficiently qualifying projections.

For example, the highlights report says: “…data records have grown longer and climate models have become more comprehensive, earlier predictions have largely been confirmed. The only real surprises have been that some changes, such as sea level rise and Arctic sea ice decline, have outpaced earlier projections.”

Legitimate climate scientists, who have published in the peer reviewed literature, could easily nit pick this sweeping statement. Countering the assertion “earlier predictions have largely been confirmed”, an August 2013 paper in Nature Climate Change concluded that computer models simulated over four times as much warming compared to reality since 1998 (note: the report does discuss the reason for this, i.e. a short-term slowdown in surface warming, in its “science supplement.”). And while the decline in Arctic sea ice has occurred faster than model projections, Antarctic sea ice has actually increased (for complicated reasons, which don’t refute global warming) while many models predicted the opposite.

Just as the report’s discussion of the success of past predictions could be better qualified or more complete, so could some of its predictions about the future.

For example, the chapter on the Northeast predicts a 60-day per year increase in the number of 90 degree or warmer days by mid-century in the northern Mid-Atlantic region. In Washington, D.C., for example, that would imply the current average of 36 90+ degree days would increase to 90 – or the equivalent of the entirety of June, July, and August.

While the number of 90+ degree days have trended upward in D.C. over time, a rather remarkable acceleration in the long-term trend would be required for this projection to become reality. For someone who’s in knowledgeable about D.C. climate trends, this projection doesn’t seem believable and the report doesn’t substantiate it.

Oversimplified statements and projections that seem over the top can do damage to an otherwise outstanding body of work – which is beautifully and innovatively presented.  Over 300 excellent scientists contributed to this report.

I’ve followed the reactions of a number of thoughtful meteorologists on Twitter today who’ve had a similar reaction.

The lack of nuance in some of the public-facing material in the report probably reflects political influence.

Capital Weather Gang’s Steve Tracton sent along this comment which I think is a fair lens through which to review the report, for those of us who are sticklers for accuracy and adherents to nuance, but appreciate the risks posed by climate change:

Understand the political rationale to go ahead with this report – motivate action to reduce emissions. So this should not be read as a purely science-based document. However, that should not be allowed to undermine the need to severely reduce mankind’s role in climate change.  However climate change ultimately unfolds, it’s reasonable to guesstimate, if not assume, that overall there will be high impact effects on societies regionally as well as globally.

Related stories:

U.S. climate report says global warming impact already severe

National Climate Assessment: The weather of the Mid-Atlantic’s future

National Climate Assessment: 15 arresting images of climate change now and in the pipeline

Greenhouse gamble: Could IPCC’s adherence to climate models backfire?

Jason is currently the Washington Post’s weather editor. A native Washingtonian, Jason has been a weather enthusiast since age 10.
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Jason Samenow · May 6