Until recently, most discussions of modern global warming have looked only as far ahead as 2100 AD. Now, new investigations by pioneering climate modelers are beginning to tell another story, one in which the legacy of our heat-trapping carbon emissions lasts not just decades or centuries but long enough to interfere with future ice ages. As science-journalist Mason Inman (2005) puts it, with only slight exaggeration, "carbon is forever."
Specialists are now investigating the long-term future of our greenhouse gas pollution with the help of a new generation of sophisticated climate models with names like CLIMBER, GENIE, and LOVECLIM. But the basics of that future boil down to one simple principle: what goes up must come down.
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Figure 1: Staghorn coral near Key West, Florida.Ocean acidification threatens these and other marine organisms that depend on acid-soluble carbonate supporting structures and shells. 2012 Public Domain Courtesy of Reef Relief. Some rights reserved.
After a delay due to slow response times in the atmosphere and oceans (Wigley 2005), global average temperatures will pivot into cooling mode as CO2 concentrations continue to fall. However, global mean sea level will still rise long after the thermal peak passes, because even though temperatures will be falling, they will still be warmer than today. Therefore, land-based glacial ice will continue to melt and the oceans will continue to expand even though Earth's atmosphere has begun to recover. Sea level will only return to today's position when it finally becomes cool enough for large, land-based ice sheets to build up again on Antarctica and in the Arctic.
In order to work out the timing of these processes in more detail, one must consider where CO2 goes after it leaves our smokestacks and exhaust pipes. Some of it will be taken up by soils and organisms but most of it will dissolve into the oceans, with between two thirds and half of our emissions perhaps going into solution during the next millennium or so (Inman 2008, Eby et al. 2009). In many computer simulations, maximum ocean acidification lasts 2000 years or more, depending on the amount of CO2 we emit in the near future. Marine species living in the polar regions and deep sea basins and trenches will be the most rapidly and severely impacted because the solubility of such gases is greatest in cold waters. But after the seas have absorbed as much CO2 as they can, roughly a fifth of our fossil carbon emissions will still be left adrift in the air (Tyrell et al. 2007, Inman 2008).
The next stage of the cleanup will proceed more slowly. As atmospheric CO2 dissolves into raindrops, the carbonic acid that it produces will react with calcite and other carbonate minerals in rocks and sediments. Over thousands of years, those geochemical weathering processes will transfer many of the formerly airborne carbon atoms into groundwater and runoff, finally delivering them to the oceans in the form of dissolved bicarbonate and carbonate ions. Meanwhile, carbonate-rich deposits on the sea floor will experience similar reactions with overlying seawater as the oceans become more acidified. This slow addition of acid-buffering substances to marine ecosystems will act much like an antacid pill that allows the seas to consume more CO2 from the overlying atmosphere. These processes are generally expected to dominate the long-term recovery for 5,000 years or so.
But even this second, lengthier phase won't remove the very last fraction of our carbon pollution. Only tens of thousands of years later, or possibly even hundreds of thousands if we burn most of our enormous coal reserves, the last remnants of our CO2 will finally be scrubbed away by even slower reactions with resistant silicate minerals, such as the feldspars found in granite and basalt. This is what University of Chicago oceanographer David Archer calls "the long tail of the carbon curve" (Archer 2005), and it will be dominated by gradual global cooling, albeit at higher temperatures than those of today.
Figure 2: Airborne carbon dioxide concentrations in a moderate emissions scenario.Note the steep initial rise, rapid climate whiplash turnaround, and slow long-term recovery over the next 100,000 years. 2012 Nature Education Reprinted with permission: Archer 2005; Archer and Brovkin 2008. All rights reserved.
Figure 3Detail of the first 2000 years of an extreme emissions scenario, showing lagged responses of atmospheric CO2 concentrations, temperatures, and sea level. 2012 Nature Education Reprinted with permission: Schmittner et al. 2008. All rights reserved.
Such long-term perspectives are not only scientifically interesting and important, they also raise new ethical questions, simply because human beings are now in the picture. Our carbon emissions will influence countless generations, as well as many species other than our own, in future versions of the world that will differ markedly from the one we know now. This realization may force us to weigh the needs of some generations against those of others.
For instance, having the Arctic Ocean become ice-free in summer may seem outlandish to us, but it may instead seem normal to people who will be born into a warmer world thousands of years from now. When the global cooling recovery sets in, the open-water ecosystems and human cultures that will by then have become dependent upon warmer climates could be threatened as the polar ocean begins to re-freeze. Will global warming seem preferable to cooling then?
Another potentially confusing situation arises when we consider that atmospheric CO2 concentrations will still be high enough in 50,000 AD to prevent the next ice age, which natural cyclic processes would normally be expected to trigger then (Figure 5; Berger & Loutre 2002, Archer & Ganopolski 2005). The next major cyclic cool period is due in 130,000 AD, by which time a moderate carbon emission will have dissipated. This suggests that preventing an extreme 5000 Gton hothouse scenario now could leave Canada and northern Europe vulnerable to being bulldozed by gigantic ice sheets in the deep future. How do we weigh the winners and losers in such a far-sighted view?
Figure 5Predicted summer insolation (sunlight intensity) values in the Arctic, showing an anticipated cooling period around 50,000 AD that would normally produce an ice age. Lingering remnants of our fossil fuel emissions may still warm the atmosphere enough then to prevent the ice age from occurring. 2012 Nature Education Reprinted with permission: M. F. Loutre. All rights reserved.
Fortunately, long-term perspectives may also suggest possible win-win situations, as well. For instance, leaving most remaining coal untouched rather than using it all up now would reduce the severity of climate change in the near-term, and would also leave large stores of burnable carbon in the ground that later generations could use as a source of greenhouse gases for the prevention of future ice ages, should they so desire.
Whichever emissions scenario we choose-be it moderate or extreme-one thing is now clear. Our influence on the climatic future of the world is geological in scope. Little wonder, then, that many scientists are now referring to our chapter of Earth history with a term coined by ecologist Eugene Stoermer-the "Anthropocene Epoch" or the "Age of Humans" (Crutzen & Stoermer 2000, Stager 2011).
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The best available evidence shows that, on the contrary, warming is likely to more or less stop once carbon dioxide (CO2) emissions reach zero, meaning humans have the power to choose their climate future.
When scientists have pointed this out recently, it has been reported as a new scientific finding. However, the scientific community has recognised that zero CO2 emissions likely implied flat future temperatures since at least 2008. The Intergovernmental Panel on Climate Change (IPCC) 2018 special report on 1.5C also included a specific focus on zero-emissions scenarios with similar findings.
Much of the confusion around committed warming stems from mixing up two different concepts: a world where CO2 levels in the atmosphere remain at current levels; and a world where emissions reach net-zero and concentrations begin to fall.
Even in a world of zero CO2 emissions, however, there are large remaining uncertainties associated with what happens to non-CO2 greenhouse gases (GHGs), such as methane and nitrous oxide, emissions of sulphate aerosols that cool the planet and longer-term feedback processes and natural variability in the climate system.
Moreover, temperatures are expected to remain steady rather than dropping for a few centuries after emissions reach zero, meaning that the climate change that has already occurred will be difficult to reverse in the absence of large-scale net negative emissions.
As a result, climate models tended to be run with scenarios of the concentration of CO2 in the atmosphere, rather than emissions, and often examined what would happen if atmospheric CO2 levels remained fixed at current levels into the future.
However, a world of constant concentrations is not one of zero emissions. Keeping concentrations constant would require some continued emissions to offset the CO2 absorbed by the land and oceans. This would amount to around 30% of current global emissions, although the amount needed would fall over time.
The figure below, adapted from a 2010 paper in Nature Geosciences by Prof H Damon Matthews and Prof Andrew Weaver, compares projected temperature changes out to 2200 under scenarios with constant concentrations (red line) and zero emissions (blue).
The Earth is currently out of thermal equilibrium, meaning more energy from the sun is being trapped by the greenhouse gases in the atmosphere than is escaping back to space. Over 90% of this extra heat is going into warming the oceans. However, as the oceans continue to warm, they will take up less heat from the atmosphere and global average surface temperatures will rise further. 152ee80cbc
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