A Glimpse at Our Possible Future Climate, Best to Worst Case Scenarios

Recently there has been widespread discussion that perhaps the Earth's climate is not quite as sensitive to increasing atmospheric CO2 as climate scientists previously believed, which would be good news, because it would give us more time to reduce human greenhouse gas emissions before the worst climate change impacts are triggered.  The case for a most likely equilibrium climate sensitivity of around 2.5°C average surface warming in response to a doubling of CO2, as opposed to 3°C, is not yet very compelling, but it is certainly a possibility.  In fact, the value could very plausibly be anywhere between 2°C and 4.5°C.

This begs the question, what might the future climate look like in best case, most likely case, and worst case scenarios?  To answer this question, we will examine how much warming we can expect under various human greenhouse gas emissions scenarios if the real-world equilibrium climate sensitivity turns out to be 2°C (best case), 3°C (most likely case), or 4.5°C (worst case).  There is a relatively small chance that the sensitivity could be lower than 2°C or higher than 4.5°C, especially if we consider very long timescales in which slow feedbacks can kick in, and the so-called "Earth System Sensitivity" may be in the range of 6°C surface warming in response to doubled CO2. 

Nevertheless, for our purposes here we will limit ourselves to the 2–4.5°C likely equilibrium sensitivity range.  But first we have to investigate at what temperatures we expect various climate consequences to be triggered.

Climate Consequence Temperature Triggers

The 2007 IPCC Fourth Assessment Report (AR4) summarizes the magnitudes of impact of various degrees of warming here, and graphically in Figure 1, relative to ~1990 temperatures (~0.6°C above late 19th Century temperatures).

fig spm.2

Figure 1: Illustrative examples of global impacts projected for climate changes (and sea level and atmospheric carbon dioxide where relevant) associated with different amounts of increase in global average surface temperature in the 21st century. The black lines link impacts, dotted arrows indicate impacts continuing with increasing temperature. Entries are placed so that the left-hand side of the text indicates the approximate onset of a given impact. Quantitative entries for water stress and flooding represent the additional impacts of climate change relative to the conditions projected across the range of Special Report on Emissions Scenarios (SRES) scenarios. Adaptation to climate change is not included in these estimations. Confidence levels for all statements are high.  IPCC AR4 WGII Figure SPM.2.  Click the image for a larger version.

Some adverse impacts are expected by the time we reach 1.5°C surface warming above pre-industrial temperatures.  For example, widespread coral mortality, hundreds of millions of people at risk of increased water stress, more damage from droughts and heat waves and floods, and increased species extinction rates.  However, by and large these are impacts which we should be able to adapt to, at a cost, but without disastrous consequences.

Once we surpass 2°C (which is internationally considered the "danger limit" beyond which we should not pass), the impacts listed above are exacerbated, and some new impacts will occur.  Coastal flooding will impact millions of people.  Coral bleaching will be widespread (exacerbated by ocean acidification), most coral reefs may not survive (Frieler et al. 2012, Kiessling et al. 2012), global food crop production will decline, and sea levels will rise by close to 1 meter by 2100.  Up to 30% of global species will be at risk for extinction.

At 3–4°C warming, widespread coral mortality will occur (at this point corals are basically toast), and 40–70% of global species are at risk as we continue on the path toward the Earth's sixth mass extinction.  Glacier retreats will threaten water supplies in Central Asia and South America.  The possibility of significant releases of CO2 and methane from ocean hydrates and permafrost could amplify global warming even further beyond our control.  Sea level rise of 1 meter or more would be expected by 2100, with the possibility of destabilization of the Greenland and West Antarctic ice sheets, which would cause much more sea level rise and flooding of coastal communities.

For further details, see our post here and the Point of No Return report (pages 23–25) by Ecofys and Greenpeace.  To summarize, we do not want to exceed 2°C warming above pre-industrial levels (even that amount of warming is very risky), and 3–4°C or more puts us at serious risk of catastrophic climate change.  Now let's examine when we can expect to reach these temperatures in various climate sensitivity and greenhouse gas emissions scenarios.

Representative Concentration Pathways (RCP) Scenarios

The Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report will begin to use the Representative Concentrations Pathway (RCP) scenarios.  Four scenarios have been developed, and named after the radiative forcing (global energy imbalance) caused by human emissions around the year 2100. 

Figure 2 illustrates how human fossil fuel CO2 emissions evolve over time in each of these RCP scenarios.  Our estimated emissions thus far are also shown in black, though while they are toward the high end of the scenarios, it is too early to say yet on which path we're headed.

Note that in RCP 3-PD, fossil fuel emissions peak around the year 2020 and decline thereafter, while they peak around 2040 in RCP 4.5, and around 2080 in RCP 6.

RCP emissions

Figure 2: RCP fossil fuel CO2 emissions scenarios through 2100.  Emissions estimates from the International Energy Agency (IEA) through 2011 in black.

Figure 3 shows how atmospheric CO2 concentrations change over time in each RCP scenario.  Note that RCP 4.5 does not reach 560 ppm (doubled CO2) despite having a 4.5 W/m² forcing (larger than the 3.7 W/m² forcing associated with a doubling of CO2) because it also includes forcings from other human greenhouse gas emissions.

RCP emissions

Figure 3: RCP scenario atmospheric CO2 concentrations through 2100.

The Best Case Climate Scenario (2°C Sensitivity)

If we are very fortunate, global surface temperatures will only warm 2°C above pre-industrial levels in response to doubled atmospheric CO2 – realistically that is the lowest equilibrium climate sensitivity we can hope for. 

Caveats: The following graphs do not include natural influences on global surface temperatures, or internal variability, but they do include all human forcings.  Note that while we focus on the equilibrium climate sensitivity in this post, short-term temperature responses are dictated by the transient climate response.  The graphs below primarily show this transient short-term response, and some additional warming that will continue to occur until the planet reaches a new energy equilibrium state.  This does not account for possible changes in the carbon cycle, like reduced ocean carbon absorption or releases from melting permafrost, for example.  These graphs only consider human emissions, and they're simple approximations (temperature = sensitivity x forcing), not climate model runs.

Figure 4 estimates the amount of human-caused warming we can expect to see from each RCP scenario in a 2°C equilibrium climate sensitivity world.  

RCP emissions

Figure 4: Estimated expected warming for each RCP scenario in a best case world with 2°C equilibrium climate sensitivity.

The best case scenario doesn't look too bad.  If we take aggressive steps to reduce human greenhouse gas emissions, we can keep global surface warming well below the most dangerous levels.  Even in RCP 4.5, where emissions don't peak until 2040, we will probably not pass the 2°C 'danger limit' in this best case scenario until after 2100, though we will be committed to about 2.4°C eventual warming once the planet reaches a new energy equilibrium.

Quite simply, if equilibrium climate sensitivity is 2°C, then we can double atmospheric CO2-equivalent levels (to 560 parts per million) before we commit ourselves to 2°C surface warming.  That will require that we take steps to transition away from fossil fuels, but at a rate which is realistically achievable.

The Most Likely Climate Scenario (3°C Sensitivity)

The most likely reality is a global surface warming of about 3°C above pre-industrial levels in response to doubled atmospheric CO2.  Figure 5 estimates the amount of human-caused warming we can expect to see from each RCP scenario in a 3°C equilibrium climate sensitivity world.

RCP emissions

Figure 5: Estimated expected warming for each RCP scenario in a most likely case world with 3°C equilibrium climate sensitivity.

The most likely case scenario is much less encouraging.  If our emissions go much beyond RCP 3-PD, we will pass the 2°C danger limit.  Even RCP 4.5 commits us to nearly 3°C surface warming by 2100, and 3.6°C once the planet reaches a new energy equilibrium.  That's square in the very dangerous and potentially catastrophic range.  Any emissions scenarios above RCP 4.5 are probably catastrophic.

The Worst Case Climate Scenario (4.5°C Sensitivity)

In a realistic worst case scenario, global surface temperatures will warm 4.5°C above pre-industrial levels in response to doubled atmospheric CO2.  Figure 6 estimates the amount of human-caused warming we can expect to see from each RCP scenario in a 4.5°C equilibrium climate sensitivity world.

RCP emissions

Figure 6: Estimated expected warming for each RCP scenario in a worst case world with 4.5°C equilibrium climate sensitivity.

This worst case scenario is an ugly one.  In RCP 3-PD we can at least limit global warming to just a bit into the dangerous range, but in all the other scenarios, we burn our way into a climate catastrophe.

Bottom Line - We Need to Reduce Fossil Fuel Emissions

There is a critical point that must be made here – the worst case scenario is just as likely as the best case scenario.  Those who argue that we can proceed under the assumption that the best case scenario is reality do so by cherrypicking convenient evidence and ignoring inconvenient evidence.

So what does this all mean?  The only way we can be certain to avoid catastrophic climate change is to take major steps to reduce global fossil fuel consumption as quickly as possible, following a similar path as in RCP 3-PD.  The more we delay, the higher the risk of climate catastrophe becomes.  And remember, we haven't accounted for account for possible changes in the carbon cycle, like reduced ocean carbon absorption or releases from melting permafrost, or slow feedbacks which may amplify global warming further in the future.

The problem is that at the moment we're moving in the wrong direction.  The rate of increase in annual global fossil fuel CO2 emissions was about 3 times faster in the 2000s than the 1990s, and the increases in 2009–2010 and 2010–2011 were two of the three highest annual emissions increases ever (data are not yet available for 2012).  At a time when global emissions need to be flattening out and approaching a peak, instead they are accelerating.  Power plants have lifespans of decades, so our decisions today lock us into a long-term emissions pathway.  We need a global agreement to turn this trend around, and fast – we can't just wake up 10 years from now and decide it's finally time to take climate change seriously.

Note also that ocean acidification is a problem which depends only on future emissions.  No matter what the climate sensitivity, ocean acidification will do major damage to global marine ecosystems if we don't get our CO2 emissions under control.

The good news is that we still have time to solve this problem.  Even in the worst case scenario we would have a chance to avoid the most dangerous climate impacts with aggressive emissions reductions, and the lower the real-world sensitivity to CO2, the more time we have.

However, that time is running out.  Those wishful thinkers who are helping to delay meaningful action to reduce greenhouse gas emissions because they refuse to consider any but the best case scenario are doing the entire world a great disservice.

At this point we only have two options – take urgent action to reduce fossil fuel consumption, or risk a climate catastrophe.  The longer we delay, the greater the risk of catastrophe becomes.  Get on board, or get out of the way.

Posted by dana1981 on Wednesday, 13 February, 2013

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