{"id":3785,"date":"2013-02-15T16:23:02","date_gmt":"2013-02-15T21:23:02","guid":{"rendered":"http:\/\/www.easterbrook.ca\/steve\/?p=3785"},"modified":"2013-02-20T12:23:09","modified_gmt":"2013-02-20T17:23:09","slug":"how-big-is-the-climate-change-deficit","status":"publish","type":"post","link":"https:\/\/www.easterbrook.ca\/steve\/2013\/02\/how-big-is-the-climate-change-deficit\/","title":{"rendered":"How Big is the Climate Change Deficit?"},"content":{"rendered":"

Last week, Damon Matthews<\/a> from Concordia visited, and gave a guest CGCS<\/a> lecture, “Cumulative Carbon and the Climate Mitigation Challenge”. The key idea he addressed in his talk is the question of “committed warming” – i.e. how much warming are we “owed” because of carbon emissions in the past (irrespective of what we do with emissions in the future). But before I get into the content of Damon’s talk, here’s a little background.<\/p>\n

The question of ‘owed’ or ‘committed’ warming arises because we know it takes some time for the planet to warm up in response to an increase in greenhouse gases in the atmosphere.\u00a0You can calculate a first approximation of how much<\/i> it will warm up from a simple energy balance model (like the ones I posted about last month<\/a>). However, to calculate how long<\/i> it takes to warm up you need to account for the thermal mass of the oceans, which absorb most of the extra energy and hence slow the rate of warming of surface temperatures. For this you need more than a simple energy balance model.<\/p>\n

You can do a very simple experiment with a Global Circulation Model, by setting CO2 concentrations at double their pre-industrial levels, and then leave them constant at this level, to see how long the earth takes to reach a new equilibrium temperature. Typically, this takes several decades, although the models differ on exactly how long. Here’s what it looks like if you try this with EdGCM<\/a>\u00a0(I ran it with doubled CO2 concentrations starting in 1958):<\/p>\n

\"EVA_time\"<\/p>\n

Of course, the concentrations would never instantaneously double like that, so a more common model experiment is to increase CO2 levels\u00a0gradually,\u00a0say by 1% per year (that’s a little faster<\/a> than how they have risen in the last few decades) until they reach double the pre-industrial concentrations (which takes approx 70 years<\/a>), and then leave them constant at that level. This particular experiment is a standard way of estimating the Transient Climate Response<\/a>\u00a0– the expected warming at the moment we first reach a doubling of CO2 –\u00a0and is included in the CMIP5 experiments<\/a>. In these model experiments, it typically takes a few decades more of warming until a new equilibrium point is reached, and the models indicate that the transient response is expected to be a little over half<\/a> of the eventual equilibrium warming.<\/p>\n

This leads to a (very rough) heuristic that as the planet warms, we’re always ‘owed’ almost as much warming again as we’ve already seen at any point, irrespective of future emissions, and it will take a few decades for all that ‘owed’ warming to materialize. But, as Damon argued in his talk, there are two problems with this heuristic. First, it confuses the issue when discussing the need for an immediate reduction in carbon emissions, because it suggests that no matter how fast we reduce them, the ‘owed’ warming means such reductions will make little difference to the expected warming in the next two\u00a0decades. Second, and more importantly, the heuristic is wrong! <\/i>How so? Read on!<\/p>\n

For an initial analysis, we can view the climate problem just in terms of carbon dioxide, as the most important greenhouse gas. Increasing CO2 emissions leads to increasing CO2 concentrations in the atmosphere, which leads to temperature increases, which lead to climate impacts. And of course, there’s a feedback in the sense that our perceptions of the impacts (whether now or in the future) lead to changed climate policies that constrain CO2 emissions.<\/p>\n

So, what happens if we were to stop all CO2 emissions instantly?\u00a0The naive view is that temperatures would continue to rise, because of the ‘climate commitment’ \u00a0– the ‘owed’ warming that I described above. However, most models show that the temperature stabilizes almost immediately. To understand why, we need to realize there are different ways of defining ‘climate commitment’:<\/p>\n