Is global warming irreversible?

Question

I have heard countless times the perils of global warming and how we should all do our fair share to save the Earth for future generations.

Here is but one example from an NPR article: "Global Warming is Irreversible, says Study"

"People have imagined that if we stopped emitting carbon dioxide that the climate would go back to normal in 100 years or 200 years. What we're showing here is that's not right. It's essentially an irreversible change that will last for more than a thousand years," Solomon says. [in an interview regarding a study published in the Proceedings of the National Academy of Sciences]

Makes trying to improve the state of affairs sound rather hopeless, doesn't it?

Others think that mitigating or reversing the effects of global warming might be possible:

At the US Department Of Energy's Ask a Scientist website (which I realize pales on peer review compared to PNAS...), some creative solutions are offered by a visitor and the scientists who reply:

Visitor: If we do not do enough to thwart Global Warming, and the oceans start to rise, could we use the effects of Nuclear Winter to offset global warming? Of course we would have to be careful about radiation and radioactive contamination by using the cleanest possible nuclear devices.

Reply by Marc Frenau: This is a good question, but fortunately you do not have to use nuclear bombs to put the dust and particles in the atmosphere. The idea is to reflect sunlight back to space and you could do this by putting lots of sulfur particles into the atmosphere. You do not need dust from nuclear explosions, you could just use rockets or supersonic transports or whatever to get the sulfate particles up to the correct height in the atmosphere....

Reply by Don Libby: Actually, some scientists (e.g. Reid Bryson at the University of Wisconsin) believe that there is sufficient dust in the atmosphere already to effectively counter any global warming effect from C02...I wonder if we could not provide the shade with less potential harm than nuclear explosions would cause, such as putting a huge tarpaulin into earth orbit to create a solar eclipse.

While I'm not sure I'd bet on manufacturing a tarp to blot out the sun, is it scientifically reasonable to doubt the irreversibility of global warming?

Couldn't there be some present or future technological countermeasure to global warming that would make it reversible, not irreversible?

Answer 1

There isn't much scientific controversy about geoengineering by injecting SO₂ into stratosphere being effective in stopping (or even reversing) global warming. However, the opponents of geoengineering call it intentional pollution, argue that results are "hard to predict".

Some peer reviewed references:

"A Combined Mitigation/Geoengineering Approach to Climate Stabilization" T. M. L. Wigley

Abstract

Projected anthropogenic warming and increases in CO2 concentration present a twofold threat, both from climate changes and from CO2 directly through increasing the acidity of the oceans. Future climate change may be reduced through mitigation (reductions in greenhouse gas emissions) or through geoengineering. Most geoengineering approaches, however, do not address the problem of increasing ocean acidity. A combined mitigation/geoengineering strategy could remove this deficiency. Here we consider the deliberate injection of sulfate aerosol precursors into the stratosphere. This action could substantially offset future warming and provide additional time to reduce human dependence on fossil fuels and stabilize CO2 concentrations cost-effectively at an acceptable level.

"Global and Arctic climate engineering: numerical model studies" Ken Caldeira and Lowell Wood

Abstract

We perform numerical simulations of the atmosphere, sea ice and upper ocean to examine possible effects of diminishing incoming solar radiation, insolation, on the climate system. We simulate both global and Arctic climate engineering in idealized scenarios in which insolation is diminished above the top of the atmosphere. We consider the Arctic scenarios because climate change is manifesting most strongly there. Our results indicate that, while such simple insolation modulation is unlikely to perfectly reverse the effects of greenhouse gas warming, over a broad range of measures considering both temperature and water, an engineered high CO2 climate can be made much more similar to the low CO2 climate than would be a high CO2 climate in the absence of such engineering. At high latitudes, there is less sunlight deflected per unit albedo change but climate system feedbacks operate more powerfully there. These two effects largely cancel each other, making the global mean temperature response per unit top-of-atmosphere albedo change relatively insensitive to latitude. Implementing insolation modulation appears to be feasible.

"Transient climate–carbon simulations of planetary geoengineering" H. Damon Matthews and Ken Caldeira

[...] Proposed schemes to reduce incoming solar radiation (e.g., ref. 3) have drawn on the climatic effect of large volcanic eruptions (e.g., Mt. Pinatubo in 1991), which inject sulfate aerosols into the stratosphere and generate global cooling of a few tenths of a degree for several years after an eruption (7). By extension, it is possible that deliberate (and repeated) injection of aerosols into the stratosphere would affect a long-term cooling that could compensate for some (or perhaps all) of the climate warming induced by anthropogenic greenhouse gases. [...]

Answer 2

Global warming due to manmade excess CO2 emissions is reversible, although if species become extinct in the meantime that's not reversible.

Left to nature, the excess CO2 will be removed from the atmosphere via the Carbonate–silicate cycle; however, at the natural rate, this will take a few million years.

Instead, although not without consequences, silicate rocks can be quarried and crushed to artificially enhance the rate of this process (converting silicates to carbonates).

See A Guide to CO2 Sequestration Science Vol. 300, pp. 1677-1678 :

serpentine or olivine rocks rich in magnesium silicates can be mined, crushed, milled, and reacted with CO2. Estimated mining and mineral preparation costs of less than $10 per ton of CO2 seem acceptable, adding 0.5 to 1¢ to a kilowatt-hour of electricity.