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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?

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    carbon taxes are in trillions of dollars, while putting sulfur into troposphere might cost as little as 100 million. – vartec Apr 11 '12 at 9:00
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    @matt_black I agree that is an interesting and worthwhile question. You might post "Is a warmer world necessarily worse?" as a question but you'd need to document it a bit. The question I've posted is merely whether it is irreversible or not. – Paul Apr 11 '12 at 9:58
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    en.wikipedia.org/wiki/… – vartec Apr 11 '12 at 10:07
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    There might be different answers here depending on timescale: human or geological. Since we are currently (despite human emissions) at a geologically low CO2 point, it seems obvious that the long term answer is yes. But what works on a human history timescale might be different or might not matter. – matt_black Apr 11 '12 at 15:32
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    But why are geological timescales more relevant than biological ones - we have only existed as a species for about 250,000 years? You are also missing the point about climate change, it is the change that causes the need for adaption. Our agriculture for instance is quite highly adapted to the climate conditions we currently have, and it will be difficult to feed the worlds population as we adapt, given that we are already having difficulty feeding everybody as it is. This is especially true in developing countries with high population densities, such as Bangladesh. – Dikran Marsupial Mar 25 '14 at 9:31
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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. [...]

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    While such geoengineering schemes address the "overall global temperature" aspect, it's worth pointing out that they would not address increased ocean acidification and would be unlikely to eliminate climate disruption (that is, global rainfall patterns, farmlands, etc.). So while they may address the question-as-stated, if it were reworded to "is global climate change irreversible?" the answer would have to be less optimistic. – Larry OBrien Apr 12 '12 at 18:23
  • @LarryOBrien: but oceans acidification isn't quite the "doomsday threat", so highly unlikely that ppl would accept trillions of dollars in taxes with excuse of preventing it. – vartec Apr 13 '12 at 9:07
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    @vartec Actually ocean acidification is pretty serious. It has huge consequences for oceanic ecosystems which are the basis of most biogeochemical cycles which we rely on to keep the planet habitable. – bon Mar 15 '17 at 23:51
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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.

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I would say climate change gets more difficult to reverse instead of using the word irreversible.

A greenhouse gas is a gas that is heavy relative to other gases. Typical greenhouse gases are methane (CH_4) and carbon dioxide (CO_2). Because these gases are heavy, they are better at trapping heat than other gases. Because the heat is trapped, the temperature increases. This is why Venus, despite being roughly twice as far from the Sun as Mercury, is several hundred degrees Fahrenheit hotter.

Now imagine the Sun's rays hitting a block of ice in the arctic. The ice will absorb some energy from the Sun's rays and reflect the remainder away. The absorbed heat will contribute to the ice melting, so that it needs less and less heat to melt. But the temperature (and surface depth) of the water is rising too high to not melt the ice, so there is now more heat where less heat is needed to continue melting the ice. This is what's known as a runaway effect due to it's exponential nature. Because the gases are heavier, they continue accumulating more heat while melting the ice at accelerating rates. This is why this past January is now the hottest January on record (last year was last January); similarly for February, March, etc.

Is it impossible to reverse an accelerating truck that is gaining mass as it drives? Perhaps not, but only if the truck first comes to a stop.

In the winters, some of the water refreezes back into ice. But recently, the amount of ice has been diminishing rather quickly. In the arctic specifically, the water is not refreezing as easily since the water is now at a higher temperature, only to go to a different part of the Earth via currents. This also contributes to the runaway effect.

Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.

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    mikey, a greenhouse gas is NOT "a gas that is heavy relative to other gases". Methane is lighter that oxygen (O2), nitrogen (N2) and argon. Greenhouse gases have a change in electric dipole moment when a bond is stretched or bent. physics.stackexchange.com/questions/101017/… – DavePhD Mar 14 '17 at 18:35
  • I'll have to read up on the electric dipole moment change via bond stretching/bending. But, it's also worth noting that the EPA considers methane a greenhouse gas. This article in Scientific American discusses the role of methane as a greenhouse gas. This article discusses why carbon dioxide has a greater impact than methane, but still counts methane as a greenhouse gas. – MPath Mar 14 '17 at 18:44
  • Yes, methane, water, and carbon dioxide are all greenhouse gases. They all change dipole moment when a bond is stretched or bent, allowing them to absorb infrared light. – DavePhD Mar 14 '17 at 18:49
  • If I wanted to read further on the subject, would you reccomend the physics route or the chemistry route? – MPath Mar 14 '17 at 18:51
  • chemistry (spectroscopy) – DavePhD Mar 14 '17 at 18:58

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