Long-term natural climatic variation

2008-08-09

in Science, The environment

One thing that only seems to be understood to a limited extent in most quarters is the degree to which humanity faces very serious long-term climatic challenges, even in the absence of human greenhouse gas emissions. This is simply because there is no reason to believe that the kind of climate that has existed for most of human history is one that is uniquely probable and likely to persist. Ongoing forces like plate tectonics, the development of carbon-rich rock through the weathering of mountains, and orbital variations (Milankovitch cycles) all have marked and overlapping effects in the long term. Paleoclimatological evidence shows a world that has differed considerably in temperatures, weather patterns, and continental layouts. Oxygen only emerged in the atmosphere 1.7 billion years after the Earth formed (though because of biological developments, rather than climatic ones). During the late Precambrian period, Earth was essentially a giant snowball. At times, evidence suggests that Antarctica hosted deciduous forests rather than an ice sheet. It seems that sometimes the forces that caused transitions from one state to another were relatively minor in and of themselves: they just pushed the overall climate system in a self-sustaining direction.

Of course, long-term natural climatic variation on the scale of hundreds of thousands or millions of years is a much less immediate concern than the consequences of humanity’s continued use of the atmosphere as a carbon dioxide dump. The latter is a real and massive immediate threat, while the latter is more of an academic consideration for the moment. That being said, it does seem important to understand that our present conditions are not some robust preference emergent from the fundamentals of the climate system; rather, it is one equilibrium among many. In a manner somewhat akin to learning that our planet/solar system/galaxy is just one of a vast multitude, this should prompt humanity to re-examine some of our beliefs about our own importance and about the stability and habitability of the planet we inhabit.

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{ 10 comments… read them below or add one }

R.K. August 9, 2008 at 4:38 pm

Here is a question I have:

You, and others, say that humanity can emit 5 billion tonnes of carbon dioxide per year and keep the climate stable. Supposedly, this is because an equivalent amount of carbon is absorbed by plants and things.

Does that mean that the planet would be cooling in the absence of human greenhouse gas emissions, since there would be a falling concentration?

Litty August 9, 2008 at 7:07 pm

Nice photo – much more abstract than usual.

Milan August 11, 2008 at 12:49 am

R.K.,

That is a good question and something I have wondered myself.

I will try to find a good answer to it.

Julia August 11, 2008 at 1:14 pm

Have you read James Lovelock? If you are interested in pursuing this, I suggest that you could do worse than to ponder his metaphor of the earth as an organism.

Milan August 11, 2008 at 1:21 pm

The Gaia Hypothesis is arguably just another manifestation of the “humans are so very special, the universe is built for us” assumption. If anything, what we know about evolution and the history of the planet suggests that life and the universe have no inherent purpose. They just carry on in accordance with probability and the functioning of physical laws.

Milan August 12, 2008 at 9:59 am

R.K.

I got an answer from one of the climatic scientists at Real Climate:

“Sorry, but the answer is no. Total emissions are currently about 9 GtC/yr, of which just over 50% is absorbed into the active land and ocean pools relatively quickly. However, this CO2 is very rapidly cycled around and so contributes to elevated CO2 levels in the atmopshere for decades to centuries. The real sink out of this active pool is the flux of CO2 into the deep ocean – which is roughly only 2 GtC/yr (with some uncertainty). Thus to maintain long-term stability in atmospheric concentrations emissions need to fall to about 2 GtC/yr – and may well be less once the climate changes start reducing the ability of the ocean to absorb CO2. (this is a 70-80% cut in emissions).

However, even then climate changes are not stabilised – warming will continue for decades past this point, and sea level rises for longer still. So further cuts in emissions to start to actual bring CO2 down may well be necessary – possibly even to zero – though this is a century-scale goal, not something required tomorrow.

5 GtC/yr is a guarantee of continuing CO2 rise, and further climate change over and above the minimum we could possibly get away with.”

Milan August 12, 2008 at 10:05 am

I sent this response:

“Your two gigatonne figure seems to be expressed in terms of the mass of carbon, whereas the figure in the Stern Review is in terms of mass of carbon dioxide equivalent:

“Stabilisation – at whatever level – requires that annual emissions be brought down to the level that balances the Earth’s natural capacity to remove greenhouse gases from the atmosphere. In the long term, global emissions will need to be reduced to less than 5 GtCO2e, over 80% below current annual emissions, to maintain stabilisation.”

From what I have read, one tonne of carbon equates to 3.67 tonnes of carbon dioxide. I know carbon dioxide is not quite the same as a mix of gasses with an equivalent radiative forcing potential, but it does seem that this makes the five gigatonne number in the Stern Review more compatible with the one you cited.

Am I correct to think that (a) the only long term sink we can rely upon for carbon removal from the atmosphere is the deep ocean (b) this presently has an absorption capacity of about two gigatonnes of carbon (about 7.2 of CO2) per year and (c) this quantity is likely to fall as the sink becomes saturated? Does it make more sense to think of a total cumulative capacity of CO2 we can emit, rather than an annual allowance?

Many thanks,

Milan Ilnyckyj”

Milan August 13, 2008 at 12:08 am

I got this in response from Real Climate:

“I think you are correct – the differences are in the unit. Stern’s number
is in the same ballpark as mine. (to go from GtC to GtCO2 multiply by
44/12 = 3.67 as you state.)”

DMG November 26, 2008 at 3:34 pm

Over a very long time span (say, thousands of years), how much capacity do human beings really have to adapt? It seems to me like any climatic situation in which agriculture is possible would be able to sustain human civilization. As long as the transitions between states happened slowly enough, they probably wouldn’t be too painful.

The danger is the risk of significant climate changes over the course of a few decades. That, or changes so large they make large-scale agriculture impossible.

Milan November 27, 2008 at 11:23 am

Who’s to say what human beings could adapt to, over the span of thousands of years? Even massive changes may prove manageable, provided they take place slowly enough.

I agree, however, that changes that made large-scale agriculture would be very hard to reconcile with the continued existence of civilization.

Gradual change also doesn’t seem to be what we should expect. According to the UK’s Met Office, business-as-usual emissions could produce 5.5 to 7.1 degrees Celsius of temperature increase by 2100. Over that kind of timeframe, adapating effectively to the kind of changes that would be happening may well prove impossible.

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