Key Climate Questions #1: How long will the effects last?

by Milan on July 14, 2011

in Climate change, Climate science, Ethics

Despite the strong consensus that human beings are dangerously altering the climate, there are many important scientific questions about climate change that remain unanswered, or where additional research would be valuable. Improved scientific understanding of these questions can help guide appropriate policy-making. This series of posts identifies what some of these questions are and provides information on the scientific work that has been done on them so far.

The various greenhouse gases (GHGs) being produced by human activities persist in the atmosphere for different lengths of time. For instance, carbon dioxide (CO2) has a longer atmospheric lifetime than methane (CH4). In addition, the effects that arise from these emissions have differing lifetimes. Some may endure for long spans of time regardless of what humanity does in the future. For instance, if the West Antarctic or Greenland ice sheets experienced significant melting, the resulting sea level rise could be expected to endure for a long span of time.

The answer to this question has important moral implications. Many future generations will live with the consequences of climate change. As such, the choices we make now affect a great many people. Exactly how many depends on how long the effects last.

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Peer reviewed science! July 18, 2011 at 7:27 pm

Eby, M., K. Zickfeld, A. Montenegro, D. Archer, K. J. Meissner and A. J. Weaver. 2009. Lifetime of anthropogenic climate change: millennial time scales of potential CO2 and surface temperature perturbation. Journal of Climate Vol 22, May 15, 2009, pp 2501-2511.

Study shows that anthropogenic CO2 and the resulting climate change are longer lived than previously thought. Even after 10,000 years, 15% to30% of the anthropogenic CO2 perturbation persists in the atmosphere. The resulting climate changes last even longer.-

A number of Canadian scientists at the University of Victoria, along with an American colleague, recently published the results of a study investigating the multi-millennial temperature response to a range of anthropogenic CO2 emissions, including very large emissions representing combustion of all known fossil fuel reserves. This study extends other research that has demonstrated that for relatively small amounts of emissions, anthropogenic CO2 may persist for a thousand years or longer, but that most of the CO2 is removed after a number of centuries. The study used the University of Victoria Earth System Climate Model to conduct a series of 10,000 year climate change simulations using either pulses of CO2 emissions, or transient CO2 emissions for a range of CO2 emissions from 160 – 5120 PgC (GtC). A number of important conclusions were drawn from the results of these experiments. First, it was shown that the long-term atmospheric CO2 response is nearly independent of the rate of CO2 emissions, confirming other recent work that has shown that it is the cumulative emissions amount that matters most. Secondly, the long lifetime of the atmospheric CO2 perturbation was clearly shown with 15-30% of emissions remaining in the atmosphere at the end of 10,000 years. However, the time to absorb a given percentage of emissions was strongly dependent on the total amount of emissions, and increased significantly for emissions greater than about 1000 GtC. For all but the lowest emission scenarios, average surface air temperature reached its maximum at least 550 years after the peak in atmospheric CO2. Furthermore, the temperature anomaly was even longer lived than the CO2 anomaly. In all experiments, at least 50% of the maximum temperature anomaly persisted after 10,000 years.

Summary courtesy of Environment Canada

Peer reviewed science! July 18, 2011 at 7:28 pm

Lowe, J.A., C. Huntingford, S.C.B. Raper, C.D. Jones, S.K. Liddicoat and L.K. Gohar. How difficult is it to recover from dangerous levels of global warming? Environmental Research Letters 4 (2009) 014012, 9pp.

Experiments with a complex global climate model add robustness to the conclusion from earlier work that it will take a very long time to reduce global temperatures from elevated levels.

As scientific evidence mounts that even small increases in global average temperature will likely lead to significant impacts, there has been growing interest in the idea of ‘overshoot scenarios’ which envision temperature, or atmospheric GHG concentrations peaking at some level above a desired target before being brought down to ‘safer’ levels. One of the key questions about overshoot scenarios is “how long would it take to lower global temperature to the target level”? A number of recent papers have provided evidence that it is much more difficult to reduce atmospheric CO2 concentrations and global temperature than previously thought. These results have been obtained with relatively simple climate models. Lowe and colleagues investigate the issue of recovery time in overshoot scenarios using, for the first time, a complex global climate model with a fully coupled carbon cycle model (HADCM3LC model). They use idealized experiments in which emissions followed SRES A2 values until being set abruptly to zero for the next 100 years at years 2012 (CO2 concentration of ~400 ppm), 2050 (CO2 concentration of ~550 ppm) or 2100 (CO2 concentration of ~1000ppm). The experiments continued until the year 2200. They find that only very low rates of decline in atmospheric CO2 concentration occur even when emissions are abruptly stopped. Rates of change for the 100 year period following the halt in emissions were -0.2 ppm/yr, -0.4 ppm/yr and -0.75 ppm/yr for the three scenarios. Temperature continued to rise out to 2200. The second part of the paper reports on experiments with a simple climate model (MAGICC) where a large number of simulations were run to explore the impact of different settings for climate sensitivity, ocean mixing rate and a carbon cycle feedback factor on global temperature reduction. These experiments were run until the year 2500 and an additional, more realistic, multi-gas emission scenario was included, with emissions peaking in 2015 and then reducing at a rate of 3% per year. The reduction in CO2 equivalent emissions in this scenario are around 47% of the 1990 value by 2050. The outcome of this large set of simulations were probability estimates of the amount of time for which global average surface temperature might exceed warming thresholds of 1.7°C, 2°C and 3°C under different model parameterizations. They find that for the multi-gas emission scenario that peaks in 2015, there is a 55% probability of exceeding a 2°C threshold above pre-industrial level, and a 30% probability that temperature would remain above that level for at least 100 years. Overall the suite of simulations provide strong evidence of the long timescales required for global temperatures to decline and remind us of how difficult it will be to return back to ‘safer’ levels following peak levels of global warming.

Summary courtesy of Environment Canada

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