I’ve written a couple of posts on climate change basics (Gases, Forcing & Surface Temperature and Energy & Projections) that described how energy enters and moves through the climate system and some physical ramifications of emitting greenhouse gases. This post will build on those in an important way by examining what is very likely to happen to the base climate system in response to increasing carbon emissions. The operative word that is used throughout is: permanency. The climate system has so far been slightly altered by our species’ emissions. Most of the effects of that alteration won’t go away for hundreds of years. As humans emit additional emissions, the effects grow.
For all intents and purposes, as far as our species is concerned, the climate system’s alteration will not go away for a long, long time – on the order of thousands of years. That’s permanency as far as we’re concerned. Or, as the paper I cite puts it: it’s irreversible. Conditions will very likely not return to those we’ve experienced in our lifetimes and in the past few thousand years for many thousands of years into the future. That’s the cold, hard scientific truth of the situation. Now, people can decide for themselves whether such irreversibility or permanency is a “good” or “bad” thing – I won’t make normative judgments for anyone else but myself. I don’t consider such a change a “good” thing. The effects I will describe here are significant, but they are only those that are easily projected. Many other effects that haven’t been considered or experienced by our species will almost certainly fall out as a result of projections discussed here. Our civil institutions are not well equipped to handle even the first-order effects, let alone the compounding influence of effects upon effects.
On a personal note, I will not describe things as ‘catastrophic’ anymore. I have hinted at this in some posts I’ve written in the past few months without much explanation. The primary reason for this is using such language simply turns people off from considering the material. I think we need more people engaged on this topic, not less, and will consider scientific results of language and framing as much as I consider climate science results (a post dealing with this specifically is in the works). That said, I will continue to not spend many resources to engage the ideologically driven skeptic community. They simply have a different worldview than I do and neither party will convince the other that their side is “correct”. One goal of this blog is to inform those who are interested and to have civil, productive discussions of peer-reviewed climate science and the political/policy implications of that science.
So, before I delve into some details, words like `permanency` and `irreversible` will be used more frequently on this blog in the future. I will not use words like catastrophic. On that note…
Susan Solomon and her coauthors published a paper in 2008 entitled, “Irreversible climate change due to carbon dioxide emissions.” The primary finding: climate change resulting from anthropogenic carbon dioxide emissions is largely irreversible for 1,000s of years after the emissions stop. As a result, atmospheric temperatures are likely to remain higher than present-day values, rainfall reductions during dry seasons are likely to occur across the planet, and sea level rise is likely to continue to occur for thousands of years even though the models they used did not include every physical process involved in the hydrologic cycle in addition to the noted lack of all first-order forcings. The study gives us an idea of the type of temperature trends we are likely to experience for the next few thousand years as well as a conservative estimate of how high average global sea level rise will be.
In similar fashion as other modeling work, Solomon et al. allow CO2 concentrations to rise, then halt suddenly at some level in the future (reflecting a dramatic shift in human behavior such as radical technological innovation, etc. I characterize this treatment of behavior as “magical” because there is never robust reasoning to adequately describe such behavior shifts). Concentrations in the study rose at 2%/year to peak CO2 values of 450, 550, 650, 750, 850, and 1200 ppmv, followed by zero emissions after hitting each peak. For reference, current annual CO2 concentrations average just over 390pppmv. What occurs after the peaks is the interesting part of this paper, as the following graph shows:
The x-axis shows time in years out to the year 3000. Pre-industrial CO2 concentrations are indicated by the dashed line near the bottom of the graph. Without any effort at emissions’ mitigation, any one of these peaks is well within the realm of possibility. What happens after each peak? An extended period of time during which CO2 concentrations remain much higher than pre-industrial levels. Concentrations remain at levels between ~300 to ~800ppmv for the next thousand years, decreasing at decreasing rates during and after they reach their respective peaks. What effect might this have on temperature? The next graph in the paper demonstrates the simulated effects:
Each curve in this graph corresponds to the emissions lines in the previous graphs. Temperatures remain at least 1°C warmer (and up to 4°C warmer) than those of the year 1800 for the next thousand years. Temperatures do not decline at nearly the rate that CO2 concentrations do in the latter part of the millenium. While CO2 concentrations remain higher throughout the period, “permanency” is evident by temperature trends through the year 3000. What does that mean for the real world? Whatever temperature shift takes place through the end of rising emissions stays in place for all intents and purposes for our species permanently.
Rising temperatures have many other effects on different earth systems, including sea levels. Here are the sea level change projections from the Solomon et al. study:
Again, each line in this plot corresponds to an emissions scenario and a temperature trace in the two previous plots. Note the y-axis on this plot: it only shows sea level rise due to thermal expansion. Any additional water entering the world’s oceans resulting from melting glaciers or land-based ice sheets are not included in this projection. Therefore, the reader can interpret this plot as a minimum of sea level rise through 3000. The greatest rise obviously corresponds to the highest emissions scenario and the highest temperature rise. 0.4m rise in the minimum projected by this study and 1.9m is the maximum. Similarly to the previous plots, sea level doesn’t decrease once emissions and temperatures stabilize. Instead, they continue to slowly increase throughout the next millenium and remain high in essence in a permanent sense.
What’s obviously inaccurate with this study is the instantaneous cessation of CO2 emissions. Many studies treat future emissions in similar fashion. How emissions decrease in the future is of course a large unknown and therefore impossible to model with high accuracy. Solomon et al. do acknowledge that their treatment of emissions is not meant to be realistic, but to “represent a test case whose purpose is to probe physical climate system changes”. The primary lesson from this paper is relevant no matter the specific future emissions pathway: the longer emissions continue at any level close to 20th century levels, the longer it will take before concentrations stop rising and begin their slow descent in a planet with full carbon sinks, and temperatures and sea levels stabilize. The point at which all of these conditions peak is, in the end, almost entirely up to us.
The policy implications of this and other studies are obvious and not-so-obvious. Among the former: the willingness of coastal residents to incur higher infrastructure and other costs in future years versus their desire to implement policies designed to mitigate their situation; the willingness of non-coastal residents to keep funding federal insurance programs that allow others to live in high-risk zones; the way in which municipalities write zoning laws: for developers or for citizens; policy development that will help populations adapt to climate change effects in their region and/or that address mitigation on a larger scale; the priority assigned to programs that may or may not generate technological innovations that would lead to adaptive or mitigative strategies at some undefined point in the future (via government or business); how to address the need that policymakers have for information that will facilitate a balanced approach between short-term gain and long-term risk management. Other implications exist, as I’m sure most readers can attest. One result of this study is clear: we have locked in a certain amount of costs just as we’ve locked in a certain amount of warming and subsequent changes in multiple earth systems.