What’s being billed as the U.S. Senate’s last chance to pass a bill that deals with climate change, the American Power Act, aims for a now-familiar target: a reduction in greenhouse gas emissions of 83% by 2050. The idea is that if the developed world can manage to reach that goal, the global goal only has to be something like a 50% cut by mid century. As has been pointed out, this will not be easy.

The authors of a recent paper in PNAS call it “a forbidding challenge.” Why? It turns out the math and underlying science are much less forbidding. Allow me to take a stab at explaining it in hopes of shedding some light into the science that’s driving all the “alarmism.” I promise it’s not that difficult.

The paper mentioned above, “The Copenhagen Accord for limiting global warming: Criteria, constraints, and available avenues” by Veerabhadran Ramanathan and Yangyang Xu of the The Scripps Institution of Oceanography does an admirable job of reducing the science to something that laypeople can handle. At least, it’s more transparent than most similar efforts that pop up in climate journals.

Here are the basics. First, there’s the concept of radiative forcing. This is how much extra heat the Earth’s atmosphere traps. It’s measured in watts per square meter. Many climatologists believe that the planet will warm up (eventually; not right away) about 3/4 of a degree centigrade for every extra watt per square meter of heat that gets trapped. Ramanathan and Xu use 0.8 °C.

Second, there’s an equation that’s been around for a while that reduces this “climate sensitivity” thing to just a few simple elements:

H′ = H0 ln [(CO2,E)/CO2,Ref]

That might look daunting for anyone without college-level math, but it’s really not that hard to get a grip on.

H′ is the radiative forcing that will lead to no more than a given temperature target, say 2 °C above preindustrial norms.

Ho is a constant with a value of 5.5 . Think of it as the background heat against which the extra radiative forcing is measured.

ln is the natural log.

CO2,E is the level of carbon dioxide (and other greenhouse gases) in the atmosphere and
CO2,Ref is the level of carbon dioxide in the atmosphere back before we started burning fossil fuels, or 280 parts per millio

Written this way, it isn’t very helpful for politicians and anyone else who wants to know how high CO2 levels can get before we commit the Earth to warming above a given amount. So some recasting of the equation is necessary:

CO2,E = (eH′/Ho) x CO2,Ref

e is the reverse function of the natural log. It’s a constant with a value of about 2.718. So now all you have to do is figure out what H′ is — and know how to use the e key on a calculator. Recall that climatologists like to use 0.8 °C per watt per square meter. Using the 2 °C target, that means H′ = 2.5 watts per square meter.

Plugging in the numbers, you get 441 parts per million. And that’s why so many experts in the field say we should try to keep things below that level.

But what if the climate sensitivity isn’t 0.8 °C per watt per square meter. What if, as James Hansen and colleagues posit, it’s more like 1.5? The lower value is basically an estimate of how fast feedbacks such as the amount of water vapour in the atmosphere will effect the global temperature. Hansen et al add in long-term feedbacks, like ice sheet coverage, and come up with the higher number. Use that value and the equation produces:

357 ppm. (Actually, Hansen’s math is a bit different, but he comes up with a very similar figure.) And we’re already at around 390 ppm.

Of course, this glosses over just how difficult it is to come up with a reliable estimate of climate sensitivity. It is an enormously complex task. But the climatology community is working on it. And the reason members of that community are already recommending we act now to reduce GHG emissions, rather than wait until we’ve eliminated some of the remaining uncertainties, is that even at the low end of the range of possible sensitivities, we are perilously close to locking in dramatic changes in the climate.

Comments

  1. #1 Mystyk
    May 18, 2010

    Looks like you left an html tag open…

  2. #2 The Olive Ridley
    May 18, 2010

    Thanks! This is easy enough to pass on to most non-scientist friends who don’t ever believe me when I say that the math is very simple.

  3. #3 Luke Silburn
    May 18, 2010

    Forgetting to close your href tags makes baby Jesus cry.

  4. #4 James Hrynyshyn
    May 18, 2010

    More specific please. They look good to me.

  5. #5 Mystyk
    May 18, 2010

    From the term “recent paper” in the second paragraph through the term “What if as…” in one of the last paragraphs is rendering as one gigantic link zone to http://www.pnas.org/content/107/18/8055.

    From “PNAS” in the second paragraph through the end of the entry, and even overflowing into the comments (including the “Posted by:” lines) all text is rendering in italics.

    This is on IE v8. Inspecting the code shows that the {/em} tag after “PNAS” is missing a closing bracket, causing my browser to not recognize the {/em} or {/a} tags (the section reads as “{em}PNAS{/em{/a}”). Some browsers auto-correct for that, but certainly not all.

    I’m using braces to illustrate because when I used html codes to render an {a} tag with proper brackets in comment 1 the SciBlogs posting engine turned the html codes into actual brackets, causing the last part of my comment to wrongly show as a link to nowhere. Maybe you can strip that out while you’re at it.

  6. #6 Jeremy Giels
    May 18, 2010

    Thank you, James, for the explanation. I’m not too good at math, but I was able to follow you along. I also read Dr. Hansen’s recent book, STORMS OF MY GRANDCHILDREN, and he used a different approach stressing watts per meter and the energy inbalance we have created by the added energy in the system.
    Why isn’t the laws of thermodynamics sited more? I think that would help in reaching the average “Joe” like myself.
    Basicly, the climate system is an energy driven system. Am I not correct? So, by our inputs, aren’t we creating more “disorder”, as you will? What you think? Thank you!

    My interpretation is we’re not creating disorder by adding energy. In fact, by trapping the heat that would otherwise radiate out more rapidly (keeping it in the Earth system longer), we’re doing the opposite. Entropy is temporarily increased. The laws of thermodynamics mandate that eventually the Earth will reach a new equilibrium and will stop warming up, just at a higher temperature than today’s. — jh

  7. #7 eNeMeE
    May 18, 2010

    http://www.pnas.org/content/107/18/8055 doesn’t have a closing tag so it’s all a link up until http://www.columbia.edu/~jeh1/2008/TargetCO2_20080407.pdf, which does have a clsing tag.

    Then there’s an italics tag that doesn’t get closed, which seems to start at ‘PNAS calls’

  8. #8 James Hrynyshyn
    May 19, 2010

    I have no idea why IE is failing to display the tags correctly. But I have moved the anchor tag to another string. Hope that works for y’all.

  9. #9 Mystyk
    May 19, 2010

    That fixed it, although you made it tougher than you had to. All you would have needed was to add a single “>” to properly close the “/em” tag after the word PNAS.

    It happens because IE doesn’t do as much error correction as Firefox or Safari. Being still the dominant browser, I would assume they don’t feel they need to put in actual work to stay ahead of the competition. IE also fails to render CSS properly (it has incomplete implementation of v1.0 even now), so this is by no means the only problem.

    Of course, when your employer is the US Government, you have exactly zero clout to tell them which browser they should be installing on all the systems.

  10. #10 naught101
    May 21, 2010

    You (and the paper you quote) also gloss over the fact that 441ppmv is only 50% likely to keep temperature increased below 2°C. Not really good odds. What would it be if we wanted, say, only a 1/10 chance of going over 2°C?

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    March 6, 2013

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