Stabilization Targets and Irreversible Climate Change

Claudia Tebaldi

By Claudia Tebaldi

Recently, I was part of a National Academy of Sciences panel tasked with writing a report on the implications of different climate stabilization targets. The report was released July 16, and you can find it, together with a good non-technical and brief summary of it online.

I guess I should start by explaining the topic. What does a climate stabilization target mean?

Most of us have heard about the threshold of 2 degrees C for global warming, which is often cited as the amount of climate change beyond which dangerous human interference with the climate system is expected to occur. In an ideal world, without any uncertainties regarding how our climate system may react to the perturbations introduced by human activities (in particular: increased greenhouse gas concentrations (GHGs)), the laws of chemistry, physics and thermodynamics would allow us to relate a temperature threshold to a specific concentration of atmospheric carbon dioxide (CO2).

We would know, say, that 430 parts per million (ppm) of CO2-like gases in the atmosphere would yield a global average temperature that is about 2 degrees C warmer than pre-industrial levels.

Unfortunately, we don’t live in a perfect world, and we don’t have 100 percent accurate information. We lack knowledge, for example, of the precise relationship between a specific atmospheric concentration of CO2 and the amount of warming that would come with it. However, we can estimate a range of temperature changes that would be anticipated for any given atmospheric concentration. This has been the more popular approach for most climate science assessments to date.

Similarly, we can also determine what range of concentrations is associated with a given temperature change target. This approach is closer to how my colleagues and I chose to frame the issue of stabilization targets, with an interesting twist.

Some very recent studies, such as Meinshausen et al., Allen et al., and Matthews et al., have shifted the paradigm for examining the relationship between GHGs and warming by linking the latter to the total amount of emissions since the start of fossil fuel burning, and extending that forward under different potential scenarios of future emissions. The emissions can thus be related to global warming outcomes, which are expressed in terms of temperature changes.

So our panel chose to frame its analysis in terms of the accumulated emissions that could be allowed while still meeting a given temperature change target.

We can also associate many kinds of impacts to a given magnitude of warming, addressing questions such as: How much more or less will it rain, if the global average surface temperature increases by 2 degrees C? How much more frequent will the most extreme heat waves be be? How will crop yields change? And what about sea level rise?

“At the end of the day, society and the natural world we depend upon care less about CO2 per se than about the warming it produces, so we might as well focus our attention on that!”

In a nutshell, this is the task that our panel set out to accomplish with its report: consider different temperature change targets (1, 2, 3, 4…degrees C of warming), and for each of them, the corresponding range of cumulative CO2 emissions that would bring us to that target on the one hand, and what the corresponding expected impacts would be on the other. We thought centering the report on levels of warming (as opposed to, say, levels of CO2 concentrations) provided a compelling framework since we were able to relate a large number of important impacts to these individual levels.

At the end of the day, society and the natural world we depend upon care less about CO2 per se than about the warming it produces, so we might as well focus our attention on that!

With this information available, policy makers can take a look at the alternative futures in front of us and decide what they are willing to commit us to, as a consequence of how much additional GHG emissions they are going to permit.

Notice that our report does not talk about which targets are desirable — that is a judgment for policy makers to make, which involves careful considerations of risk. Also, we were not tasked with considering what it will take in terms of energy and technological advances to attain the different stabilization targets.

You can find a very simplified list of findings here about the impacts of different climate stabilization targets (i.e. levels of global warming). They include, for each degree Celsius of warming:

  • 5-10 percent changes in precipitation in a number of regions

  • 3-10 percent increases in heavy rainfall

  • 5-15 percent yield reductions of a number of crops

  • 5-10 percent changes in streamflow in many river basins worldwide

  • About 15 percent and 25 percent decreases in the extent of annually averaged and September Arctic sea ice, respectively

The report also reinforces a key aspect of climate change that, I'm frequently surprised to find, still escapes a lot of people’s attention: its irreversibility.

This has to do with what sometimes is described as the bathtub effect, an analogy meant to highlight the fact that a large portion of our emissions of CO2 (about 45 percent of them) remains in the atmosphere long after they are emitted, and thus emissions accumulate over time. (When I say “long after,” I really mean it: we are talking about centuries and millennia, not just decades).

They behave like the water filling up a bathtub whose drain, even if not completely stopped, is very weak, so that the flow of water leaving the bathtub is much slower and smaller than the faucet's flow.

That’s why choosing a desired maximum level of concentrations has direct consequences for how much additional emissions we produce and for how long. The level of water in the bathtub represents the CO2 concentration in the atmosphere. If we are to set a maximum level for the water in the bathtub, and we already have poured a large amount of it in there, there is only so much more that we can afford to add.

And if the CO2 does not go away, temperatures won’t return to their preindustrial levels either. In fact, temperatures will keep on increasing for about a century after CO2 concentrations are stabilized. A rule of thumb is that as much warming that we experience while CO2 concentrations increase still lies in the pipeline when CO2 concentrations become constant. This warming will be realized eventually over the decades and centuries as the oceans slowly warm up. So the actual warming at a given stabilization target will not only be impossible to erase, but will manifest itself for many decades to come after the stabilization.

Consider too that committing our world to warmer temperatures over long time scales of centuries to millennia will put future human populations at greater risk of large, dramatic changes that have the potential to further amplify the warming and cause huge societal disruptions, such as the melting of the ice sheets of Greenland and Antarctica, which would cause a massive increase in sea levels, or the release of the CO2 reservoirs in the permafrost and the bottom of the ocean.

These potential changes would make the human footprint on our planet as important and consequential as geological scale changes. When scientists say that we are entering the “Anthropocene epoch,” this is what they have in mind. As our report states:

“Emissions of carbon dioxide from the burning of fossil fuels have ushered in a new epoch where human activities will largely determine the evolution of Earth’s climate. Because carbon dioxide in the atmosphere is long lived, it can effectively lock the Earth and future generations into a range of impacts, some of which could become very severe. Therefore, emissions reductions choices made today matter in determining impacts experienced not just over the next few decades, but in the coming centuries and millennia.”

Even if you don't care about what happens in a thousand years, you should still consider the fact that each year that goes by without substantial changes in the way we emit CO2, we lock ourselves into an increasingly warmer future, with no turning back once we realize we don’t like it that hot, or it is very costly to adapt to it.