Climate Change: Irreversible

Jan 28th, 2009 | By | Category: Environmental, Featured Articles, Lead Article

As I’ve written about before, the carbon humanity has already added to the atmosphere is already at a level likely to cause devastating climate change in the coming years and decades. Nor have any political efforts succeeded at even reducing the pace of increases in global carbon emissions.

The optimistic among us assume that, eventually, new technology or new political movements will stop carbon release into the atmosphere. One of the comforting assumptions about climate change is that the effects of humans putting carbon into the atmosphere can be reversed. Plants remove carbon from the atmosphere, right? So, if we just stop adding more, eventually carbon dioxide levels in the atmosphere should drop, and the adverse climate changes should reverse.

Nope, at least not according to Irreversible climate change due to carbon dioxide emissions in this week’s PNAS.

Now, that’s a provocative title. The authors made such a claim very carefully. (I suggest reading the paper in whole, I’ll just summarize it here.)

First, they only considered climate changes that are:

1. going on now–not predicted for the future in a computer model, but instead directly observable today.

2. by direct evidence, caused by human activities.

3. caused by a basic physical process that is well understood by science.

4. projected by multiple and reliable computer models to worsen with increasing atmospheric carbon dioxide levels.

That is a very strict set of criteria. As far as irreversible, the authors considered effects that would remain around until at least the year 3000, even if humans totally stopped adding new greenhouse gases into the atmosphere today.

Well, what made the cut?

1. Atmospheric CO2 levels are staying high, no matter what.

When CO2 is dumped into the atmosphere, only about 20% remains in the air. About 80% dissolves into the oceans, becoming carbonic acid in the process.

CO2 (carbon dioxide) + H20 (water) <--> H2CO3 (Carbonic Acid) <--> H+ + HCO3-

This absorption of carbon dioxide into the oceans is reversible, but only in the surface water. Since the deep waters of the ocean are only rarely overturned to the surface, this happens very slowly. How slow can be observed by comparing the ratio of radioactive carbon-14 CO2 to non-radioactive carbon-12 CO2 in the air versus the ocean water, giving us a very good sense of the pace.

The results:
If we stopped adding carbon to the atmosphere right now, even a thousand years from now we still wouldn’t return back to pre-industrial levels. In fact, we can expect that carbon levels would only drop by about 60% from the peak. Since human carbon emissions have been growing by about 2% a year since the industrial revolution, this peak is still rocketing upwards.

This is nasty business.

Acidification of the oceans causes a whole mess of problems, including the potential collapse of the entire aquatic food chain, or the loss of all shelled ocean life forms (as acidic ocean water dissolves away shells.)

2. Global surface temperatures are going up.

The persistence of the carbon means global average surface temperatures will remain elevated for millennium after the last bit of human-produced carbon is added. All that carbon keeps trapping solar energy. If you recall from my earlier writing, increased temperatures alone are going to cause serious problems for food crops, regardless of water supply.

3. Ocean levels are going persistently higher

Ocean levels are rising will stay that way, not just by melting of glaciers but just by simple thermal expansion of the existing oceans. Warm water takes up more volume than cold. The expansion in ocean volume we can expect from even very modest increases in average surface temperature are likely to cause serious problems for coastal areas worldwide, with ocean levels staying about 1-2 meters higher long after we’ve stopped adding CO2 to the atmosphere.

4. Precipitation patterns are going to change.

Rainfall is going to go down in the Mediterranean, southern Africa, and parts of southwestern North America and become less predictable everywhere.

This is a damning and bleak report–made all the more so by the obvious care and caution that went into the analysis. I’m taking it seriously. You should too.