There will come a time when most of our coastal cities will look a bit like Venice at high tide – we have committed ourselves to this future. Photograph © Andrea Pattero/AFP/Getty Images
Those who follow the discussion of climate change over the years have become used to the concept of “committed warming”. That is the warming of the planet that is going to occur in the future because of greenhouse gases we have already released into the atmosphere. To understand what is meant here, think of the CO2 and other GHG we release as a single extra log we place into the fireplace on a cold day. Even if the fire is already lit, the new log takes some time to reach its combustion temperature, and then as it burns it gives off heat to the room. Sitting in front of the fire, we become warmer. That extra warming we experience over the next hour or so was committed to as we placed the log on the hearth. Short of wrestling the log out of the fire and getting it out of the house or into a bathtub of water, we are committed from that moment of adding it to the fire to some additional warming over what would otherwise occur.
When we take actions that commit us to warming in the next couple of hours, the concept of committed warming is trivial. When we actually want to be warmed, and take the action deliberately to cause warming the concept of commitment is welcome rather than threatening. But when our actions are undertaken with no intention of causing warming, as when we release CO2 into the atmosphere, when the warming is not desired, and when the duration of commitment takes years or decades, committed warming is definitely alarming. The addition of CO2 and other GHG to the atmosphere takes decades to result in its full warming effect because the planet is a very large body requiring substantial extra heat to increase its temperature a small amount, and the added CO2 merely raises the R value, insulation value, of the atmosphere. Adding CO2 to the atmosphere does not make the sun burn hotter; it causes a tiny shift in the thermal properties of the atmosphere such that some extra heat slowly accumulates in the planet system – heating of the oceans is the major reason for the slowness of the response. In their Fourth Assessment (AR4) in 2007, the IPCC reported that committed warming at the year 2000 was about 0.6oC by 2100 with further warming after that. In other words, at 2000, we had already seen about 0.8oC warming since preindustrial times, and were committed to almost as much again (0.6oC) by 2100 even if we had switched off all releases of GHG at that time. Given that we continued to release GHGs, it follows that we now expect an even greater increase in temperature by 2100. On current trends we are expecting 4-5oC by century end.
Commitment does not stop with warming. Every aspect of climate change is subject to similar lags in the huge natural system which is Earth, and the particular set of lag times determines the extent of commitment for each aspect of climate change. Ocean acidification, which I separate from climate change because it is due entirely to CO2 and unaffected by other GHGs, also has significant commitment because of the immense lag times in equilibration of chemistry of different parts of the ocean. Some new results on committed change in sea level were reported this month.
Sea level began rising rapidly as the Pleistocene came to an end, but has been remarkably stable for the past 6-7 thousand years, but is now starting to rise again. Image © Wikipedia Commons
Anders Levermann of the Potsdam Institute for Climate Impact Research in Germany, with six colleagues from US, Canadian and European institutions has a manuscript in Proceedings of the National Academy of Sciences, USA, that was published on-line on 15th July, 2013. Their title, The multimillenial sea-level commitment of global warming, reveals that they are talking about commitment over tens of thousands of years. All of their simulations went 2000 years into the future and several went 5 million years out. Committed sea level rise is very long-term.
Sea level rises as temperature rises for four distinct reasons, and each has its own pattern of time lags and therefore its own pattern of commitment. Water expands very slightly as it warms, so the first cause of sea level rise is simple thermal expansion of the ocean. Melting of glaciers contributes ‘new’ water to the oceans and is the second cause of sea level rise. The melting of the ice pack on Greenland and Antarctica also contribute to sea level rise, and because the two locations behave so differently Levermann and colleagues treat them as separate causes of sea level rise.
So what do they find? First, the increases due to thermal expansion and glacier melt equilibrate relatively rapidly (100s of years) while the melting of the Greenland and Antarctic ice sheets continues for many thousands of years after temperature has increased. For example, if we somehow manage to meet the international commitment made at the Copenhagen climate conference to keep temperature at 2100 below a 2oC rise from 20th century levels (a possibility that gets more remote every week we dither), sea level will probably rise about 4.6 meters between now and the year 4000, but it will ultimately rise about 12 meters before finally equilibrating to this new, 2oC warmer global temperature. I think the staggeringly long duration of these times to equilibrium (even the 1000 years or so for thermal expansion is a long time relative to human lives) should finally jolt people into realizing what committed climate change really is.
This figure shows the expected increase in sea level during the first 2000 years for total temperature increases up to 4oC. Panel E is the total increase expected, while A represents the increase due to thermal expansion, B, the increase due to glacier melting, C, the increase due to melting of the Greenland ice cap, and D, the increase due to melting in Antarctica. Effects in A and B are largely completed within this time period (i.e. no further commitment), but effects in C and D provide substantial further committed change over several millennia. Figure from Levermann et al, PNAS, July 2013.
Humans are used to making commitments, sometimes to be kept, sometimes to be abandoned. The commitments are made for weeks, months or years. They are not immutable. But when the world is committed to climate change that commitment is not something we can cancel, and it may be a commitment that will take many centuries to play out. Sea level has been remarkably constant for the past 6-7000 years or so. Human civilization has developed mostly during that 6000 year span. Our cities have often been built on coasts, or at the mouths of rivers. At the present time, 2.6 billion people, 37% of the global population, live within 100 km of a coast, and 19 of our 25 megacities (>10 million people) are coastal. Without thinking about sea level at all, we have warmed the climate, and even if we manage to stop the warming at 2oC, we have committed ourselves to flooding that will impact millions of people and trillions – maybe gazillions – of dollars’ worth of built infrastructure. The sea may rise slowly and for many decades to come, but it is going to rise, and we cannot now stop it. That is a real commitment!
Despite its very long-term time-frame, the Levermann paper has attracted some press. And so it should. Those small island nations we thought might be drowned by climate change WILL be drowned. The question is simply how long will it take. That third of Bangladesh that might be drowned, the one with the extensive agricultural lands that feed the people – it WILL be drowned. And south Florida – we will finally have a new Atlantis under the waves. Again, the only question is WHEN. Writing on Climate Central, Ben Strauss has produced an assessment of impacts on US cities. He provides an interactive map that allows you to choose our current trend in rising GHGs or deep cuts to emissions and to choose any decade up to 2100. The map shows which US States will have cities flooded, provides lists of cities, estimates of size of population impacted, and links to flooding maps for specific cities. This site irritates by using non-metric measurements, but maybe the knowledge that under current trends, by 2100 we will have seen 23 feet of sea level rise will register more with many people than a reference to 7 metres.
I’d love to find similar information for Canada, and especially for our Arctic coast, because I think it is rapidly becoming time to look carefully at what we may be creating up there. Opening up the North to development sounds like a lot of fun, but the changes in temperature and sea level that are happening will have profound environmental effects, even before we let our mining multinationals loose, with lax regulations and little enforcement of those that exist. Perhaps Canada’s new Minister of Environment, Leona Aglukkaq, will be more motivated than Peter Kent was to look into what we might be getting ready to do up there.
Leona Aglukkuq, Canada’s new Minister of Environment. Notice she already has the lie measurement gesture down pat. Must have taken lessons from Peter Kent, or perhaps Stephen Harper. Photo © Fred Chartrand/Canadian Press
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