Physics vs. Biology–Thinking Out Loud

I would like to try and develop a topic on the fly, writing down my thoughts pretty much as they occur and seeing where they lead. This topic is the relative weight placed on physics in determining the extent of global warming, its impacts and the constraints on our options to deal with it, and the relative discounting of biological processes that may make the inputs to physical equations harder to determine.

In this I need to acknowledge the impact of my recent reading of work by and about Freeman Dyson. Dyson is a theoretical physicist and quite possible the second smartest person on the planet. (Possibly the smartest, Stephen Hawking, is on the other side of the fence from Dyson regarding global warming. Hawking is far more concerned about it than Dyson.) However, Dyson worked in the field of climate science for 15 years and has consistently made the point that while we roughly know the relative sizes of the major carbon sinks (ocean absorption, ocean plant life, atmosphere, vegetation and topsoil), at least to the ‘right number of zeroes’, we don’t know enough about how they interact.

Carbon Sinks and Sources

Most criticism of climate models involves uncertainty about cloud cover and aerosols. But attempts to respond to this criticism has been about doing better physics. I submit that doing better biology would be a precursor to getting better answers. Clouds and aerosols have biological properties as well as influencing outward radiation at certain frequencies. Those biological properties may well be important. Vegetation, as Dyson recently pointed out, has increased by 7% globally in recent years. This was not something the physics-based scientific community anticipated. More importantly, I don’t see anybody discussing the possible effects of significantly more vegetation. That’s a lot of photosynthesis happening.

Similarly, vast changes in land use and land cover obviously change the albedo of the earth’s surface. But perhaps too obviously. Are we convinced that albedo is the only, or even primary change that should be considered? (To be fair, physicists also look at the vast vertical columns of air that are displaced by such changes–but even that begs the question, when we change the properties of the land, we are changing the biology–the plants that we grow for food change the climate and the topsoil as well.

If this is not quickly shown to be arrant nonsense, I hope that people will engage with this. Certainly I would like to see papers showing that the biology of the biome is appropriately considered in the delicate dance of climate change. But I also would like to hear thoughts on how it could be better integrated into our discussions. Of course, then will come the chemists…


6 responses to “Physics vs. Biology–Thinking Out Loud

  1. We were talking about the role of vegetation in changing the heat capacity of the earth’s surface 45 yrs ago. This was forgotten in the new climate science.

  2. Those biological properties may well be important. Vegetation, as Dyson recently pointed out, has increased by 7% globally in recent years. This was not something the physics-based scientific community anticipated.

    I’m confused why you would suggest this. It is well know that about one-quarter of our emissions have gone into the biosphere. Our total emissions are about 550GtC. The biosphere has a total mass of maybe 2000GtC. So, 130GtC extra is not far off 7%. No great surprises there.

    More importantly, I don’t see anybody discussing the possible effects of significantly more vegetation. That’s a lot of photosynthesis happening.

    I assume you’re suggesting that if we were to stop emitting CO2, the biosphere would continue to take up extra CO2 at a rate that drew down our emissions quite rapidly. A few thoughts on this.

    The natural sinks have roughly taken up 55% of our emissions, the biosphere taking slighlty more than the oceans. Why has this fraction stayed constant? If, in future, the biosphere could draw down our emissions rapidly (say at more than 2GtC/yr) why didn’t it absorb all our emissions when the annual rate was low and then draw down a decreasing fraction as our emissions increased? The current thinking – I believe – is that the fast cycle equilibrates quickly. If we emit 1GtC then the quasi-steady state is for about 1/4 to go into the oceans, 1/4 into the biosphere, and 1/2 to stay in the atmosphere. If so, if we were to stop emitting, the system would already be in a quasi-steady state and the draw-down rate would be slow.

    A few other numbers. Let’s imagine the above is wrong and that the biosphere could continue to draw down atmospheric CO2 at about 2GtC/yr. Here’s problem number 1. There’s a carbon cycle, the biosphere both absorbs and emits CO2 (currently about 60GtC/yr in both directions). If it’s growing, however, the amount emitted next year will exceed the amount absorbed this year. Therefore, it gives back some of what it takes in, so it wouldn’t draw it down at 2GtC/yr since some of that would be re-emitted next year. Also, if the atmospheric concentration is dropping, Henry’s Law tells us that the ocean will also outgas some of their excess CO2, again reducing the rate at which the atmospheric concentration would be drawn down.

    Some other numbers. The total mass of the biosphere is about 2000 GtC. We have the potential to emit an addional 500GtC by the mid-21st century, bringing our total emissions to about 1000 GtC; about half of the mass of the biosphere. If you want the biosphere to make a massive dent in these emissions, it implies that it would grow by maybe 50%. How fast can it do that?

    If you really want to get an idea of what scientists think will actually happen you can play around with the carbon cycle calculator. The default setting is a 1000GtC pulse which then produces an atmospheric CO2 concentration of about 700ppm. It certainly drops quite quickly initially, but still takes 500 years to get back to 400ppm, and by 1000 years it’s at 370ppm. By 100000 years it still hasn’t quite got back to 280ppm.

  3. ATTP, I have seen discussions where many said that vegetation would not increase as a result of increasing CO2, saying other constraints on growth were more significant.

    I don’t know if you’re familiar with Bart Verheggen’s blog (if not you should investigate it), but I asked him the following question:

    “What I always read is that the carbon sinks of the world absorb about half of our emissions of CO2. Seems natural, logical.

    What I don’t get is why the sinks can absorb 4 million metric tons of carbon when it is half of an annual emission of 8 million tons (as in 2005), but fail to absorb the same 4 million tons when it is 100% of the total (as in 1970).

    Can you explain a bit for me?”

    And received this in reply, which I believe is quite similar to what you are saying:

    “Good question.

    The ocean carbon sink depends on the concentration difference w.r.t. pre-industrial. The land sink is more complicated and depends besides on this same conc difference also on changes to the growing season, excess nitrogen, rainfall, etc. In general, the land sink is much more variable than the ocean sink.

    If the emissions would suddenly double, the sink would thus not double in lockstep, but would lag behind, depending on how the concentration develops. If the emissions are rising steadily, and so does the concentration, then the time evolution of both may look similar by coincidence, but physically it’s the conc difference with pre-ind that’s the driving force behind the sink strength.

    In the 70s the CO2 conc was 330 ppm, i.e. 15% above pre ind. In the 2000’s it was 380 ppm, 26% above pre-ind, i.e. almost double in terms of conc difference. And the sinks as a result also approximately doubled. The emissions may have approximately doubled over that period as well coincidentally.”

    He offered references:

    Some textbook chapters that are on-line: (quite heavy on the chemistry) (Global Carbon Project)

    IPCC AR5 Ch 6:

  4. I hope that my post does not give the impression that I think the physics-dominated community ignores biology. I don’t believe that’s the case. I just don’t think they pay as much attention to it as it deserves.

  5. Pingback: The Lukewarmer's Way

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