Scientific American: What Is the Right Number to Combat Climate Change?

[OKIsItJustMe]

http://www.scientificamerican.com/article.cfm?id=right-number-to-combat-climate-change

November 30, 2009

What Is the Right Number to Combat Climate Change?

Is there a safe level of carbon dioxide in the atmosphere to prevent "dangerous anthropogenic interference" in the climate?

By David Biello

This December, world leaders will meet in Copenhagen to add more hot air to efforts to combat climate change. That is so because although the impacts humanity would like to avoid—fire, flood and drought, for starters—are clear, the right numbers to halt global warming are not. Despite decades of effort, scientists do not know precisely what temperatures or greenhouse gas concentrations in the atmosphere constitute a danger.

When it comes to defining the climate's sensitivity to forcings like rising atmospheric carbon dioxide levels, "we don't know much more than we did in 1975," says climatologist Stephen Schneider of Stanford University, who first defined the term climate sensitivity in the 1970s. "What we know is if you add watts per square meter to the system it's going to warm up."

Greenhouse gases add those watts by acting as a blanket, trapping the sun's heat; they have warmed Earth by roughly 0.75 degree Celsius over the last century. Scientists can measure how much energy greenhouse gases now add (roughly three watts per square meter), but what eludes precise definition is how much other factors—the response of clouds to warming, the cooling role of aerosols, the heat and gas absorbed by oceans, human transformation of the landscape, even the natural variability of solar strength—diminish or strengthen that effect. "We may have to wait 20 or 30 years before the data set in the 21st century is good enough to pin down sensitivity," says climate modeler Gavin Schmidt of the NASA Goddard Institute for Space Studies (GISS).

Despite all these variables, scientists from Svante Arrhenius to those on the United Nations Intergovernmental Panel on Climate Change have noted that doubling preindustrial concentrations of CO2 in the atmosphere from 280 parts per million (ppm) would likely result in a world with average temperatures roughly 3 degrees C warmer.

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[ananda]

1. Erring on the side of caution sounds good to me.

nt

[kestrel91316]

2. 350?

[OKIsItJustMe]

3. That will “require a massive shift in human society”

…

Keeping atmospheric concentrations of greenhouse gases below 550 ppm, let alone going back to 350 ppm or below, will not only require a massive shift in human society—from industry to diet—but also, most likely, new technologies, such as http://e360.yale.edu/content/feature.msp?id=2197">capturing CO₂ directly from the air. "Air capture can close the gap," argues physicist Klaus Lackner of the Earth Institute, who is looking for funds to build such a device.

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[GliderGuider]

4. They're asking what number is safe, not what number is possible.

We can safely say that 280 is safe. We might be able to say 350 is safe, though the higher you go the greater the doubt becomes.

IMO neither of those numbers is achievable on a voluntary basis within the next 50 years.

[OKIsItJustMe]

6. In the next 50 years, 350 seems extremely unlikely to me. 280 seems to be out of the question.

[GliderGuider]

10. 425 with a complete coal phaseout by 2030....

Why don't we just wish for rainbow-coloured, solar-powered fairies to fly out of our butts?

Just when I was feeling all optimistic and positive after a great weekend retreat.
Reality sucks.

[OKIsItJustMe]

11. Sorry to be a downer…

Time to go back to that retreat…

[mn9driver]

5. For right now it is enough to say it must be lower.

And in order to level it off and start it heading down, CO2 emissions have to be reduced.

A lot. Like, by 80 percent.

Then in 80 years or so, we can get a handle on what a "good" stable CO2 atmospheric concentration will be. Whatever it turns out to be, the planet will not reach it for a couple hundred years. The storage part of the carbon cycle is very slow, and has been working at max capacity for around 150 years.

[OKIsItJustMe]

7. We may not have the luxury of “80 years or so”

If we have kicked off certain feedbacks… our efforts to cut our contributions to atmospheric greenhouse gas concentrations may be fruitless.

(Remember, the ecosystem has warmed dramatically in the past with no help from us.)

[joshcryer]

13. +1 agree completely.

The fact that ice flow observations have blown away our models should be enough for us to stand back and reassess.

[The2ndWheel]

8. If we number it, we think we can measure it

If we measure it, we think we can control it. As is brought up, the devil is in the variables. Everything we do, or don't do, creates different variables. For example, all the talk about decreasing emissions to 1990 levels by 2050 don't mean much, since there will be more people on the planet than there were in 1990, and a far different global socio-economic reality than we had in 1990. It's difficult to predict what we'll be doing 40 years from now, especially with something as complex as this topic. The only safe thing to say is that it'll be a trade off; some things will be better, others will be worse, depending on your definition of better and worse, which is basically the reality we've been living in for a very long time.

[GliderGuider]

9. +1

:thumbsup:

[joshcryer]

12. The fact that we lack understanding of dynamical ice flow says we don't know the number.

2.0C was a nice round number that looked to be reachable. We won't be able to reach it. So the goalposts will be moved. 2.5C, 3.0C.