Mike Mann: Irreversible Climate Change a Few Years Away

March 26, 2014

Above, Mike Mann discusses his new piece in Scientific American, which outlines critical thresholds in climate warming in the not-too-far-off future.

Scientific American:

Most scientists concur that two degrees C of warming above the temperature during preindustrial time would harm all sectors of civilization—food, water, health, land, national security, energy and economic prosperity. ECS is a guide to when that will happen if we continue emitting CO2 at our business-as-usual pace.

I recently calculated hypothetical future temperatures by plugging different ECS values into a so-called energy balance model, which scientists use to investigate possible climate scenarios. The computer model determines how the average surface temperature responds to changing natural factors, such as volcanoes and the sun, and human factors—greenhouse gases, aerosol pollutants, and so on. (Although climate models have critics, they reflect our best ability to describe how the climate system works, based on physics, chemistry and biology. And they have a proved track record: for example, the actual warming in recent years was accurately predicted by the models decades ago.)



I then instructed the model to project forward under the assumption of business-as-usual greenhouse gas emissions. I ran the model again and again, for ECS values ranging from the IPCC’s lower bound (1.5 degrees C) to its upper bound (4.5 degrees C). The curves for an ECS of 2.5 degrees and three degrees C fit the instrument readings most closely. The curves for a substantially lower (1.5 degrees C) and higher (4.5 degrees C) ECS did not fit the recent instrumental record at all, reinforcing the notion that they are not realistic.

To my wonder, I found that for an ECS of three degrees C, our planet would cross the dangerous warming threshold of two degrees C in 2036, only 22 years from now. When I considered the lower ECS value of 2.5 degrees C, the world would cross the threshold in 2046, just 10 years later [see graph on pages 78 and 79].

So even if we accept a lower ECS value, it hardly signals the end of global warming or even a pause. Instead it simply buys us a little bit of time—potentially valuable time—to prevent our planet from crossing the threshold.

Cautious Optimism
These findings have implications for what we all must do to prevent disaster. An ECS of three degrees C means that if we are to limit global warming to below two degrees C forever, we need to keep CO2 concentrations far below twice preindustrial levels, closer to 450 ppm. Ironically, if the world burns significantly less coal, that would lessen CO2 emissions but also reduce aerosols in the atmosphere that block the sun (such as sulfate particulates), so we would have to limit CO2 to below roughly 405 ppm.

We are well on our way to surpassing these limits. In 2013 atmospheric CO2 briefly reached 400 ppm for the first time in recorded history—and perhaps for the first time in millions of years, according to geologic evidence. To avoid breaching the 405-ppm threshold, fossil-fuel burning would essentially have to cease immediately. To avoid the 450-ppm threshold, global carbon emissions could rise only for a few more years and then would have to ramp down by several percent a year. That is a tall task. If the ECS is indeed 2.5 degrees C, it will make that goal a bit easier.

41 Responses to “Mike Mann: Irreversible Climate Change a Few Years Away”

  1. […] Above, Mike Mann discusses his new piece in Scientific American, which outlines critical thresholds in climate warming in the not-too-far-off future. Scientific American: Most scientists concur tha…  […]

  2. climatebob Says:

    When Mann talks about the climate and two degrees is a threshold we should not cross. There is a forty year lag in temperature rise while heat goes into the oceans so we are only now getting the weather from the CO2 of 1970.
    There is a long way to go yet. So far 0.8 C increase has brought extreme weather event which are uncomfortable but we can live with. At some point in the near future food production on the scale and price we are used to will be under extreme difficulty and it is not going to get better. Thats when revolution starts.

    • climatebob Says:

      Good point. I think they lie in the area of what we don’t even know that we don’t know.

    • Thanks Mike. Just wanted to high lite something from that.
      “what seems clear is that any strong increase in demand will quickly tell us that we are at the end of growth — particularly when we have a global derivatives debt that is almost 20 times the size of the world economy. These derivatives are basically a complex form of financial gambling and future risk-taking. They are part of our hugely problematic debt-based money system which seemingly insists on infinite growth; a perverse concept which violates the limits of our biosphere.”
      Maybe the collapse of the debt bubble will signal the end of laissez faire capitalism. Compound growth is inherently in conflict with declining resources.

      • We’re at the end of growth of some things.  Crude oil seems awfully resistant to increased extraction; higher prices are producing demand destruction faster than more drilling.

        Some other energy sources are almost completely untapped.  My favorite:  the US has used about 0.5% of the energy in the uranium mined and used for light-water reactors.  Canada’s HWRs get about 1%.  That leaves a lot of energy that we’ve already done the hardest work to get.

  3. […] 2014/03/26: PSinclair: Mike Mann: Irreversible Climate Change a Few Years Away […]

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