Why Climate Projections Have Error Bars

Lisa MooreThis post is by Lisa Moore, Ph.D., a scientist in the Climate and Air program at Environmental Defense Fund.

In 1992, the world’s nations gathered to negotiate the United Nations Framework Convention on Climate Change. The 192 nations that ratified this treaty – including the U.S. – agreed to the following objective:

[T]o … prevent dangerous anthropogenic interference with the climate system… within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a sustainable manner.

The definition of "dangerous" is a social and political judgment that is informed by science. But even if we all agreed on which outcomes we wanted to avoid, scientists couldn’t say precisely how much we have to cut emissions to achieve these outcomes. We have good best estimates, but there’s always a degree of scientific uncertainty.

Here’s why.

There are essentially three layers of uncertainty in quantifying the impacts of climate change:

  • How emissions affect greenhouse gas levels
  • How greenhouse gas levels affect climate
  • How climate change affects Earth’s systems

How emissions affect greenhouse gas levels

Even if we knew exactly what emissions would be in the future, there still would be uncertainty about future greenhouse gas levels.

Plants and the oceans currently act as "carbon sinks", offsetting some of our CO2 emissions. But we don’t fully understand how they will respond as emissions climb. As I wrote in a previous post, it appears that oceans are taking up a smaller fraction of our CO2 emissions than they did a few decades ago, and we’re not sure why.

We also don’t know how much CO2 and methane will be emitted from melting permafrost. Climate feedbacks such as this can greatly accelerate warming.

For these reasons and more, scientists can estimate how greenhouse gas levels will change for a given emissions scenario, but there is uncertainty around that estimate.

How greenhouse gas levels affect climate

Even if we knew exactly what greenhouse gas levels would be in the future, there still would be uncertainty about how climate would change as a result. This is true at the global level, and even more so at the regional level.

Climate feedback processes affect how much climate changes in response to greenhouse gas levels. The degree of impact on climate, called climate sensitivity, is usually quantified as the amount of warming from a doubling of CO2 levels. Climate sensitivity is estimated with a measure of uncertainty because climate feedback processes that affect it cannot be precisely measured.

The latest IPCC report (WG1, Chapter 10) estimates that global climate sensitivity is "most likely" 3.0°C (5.4°F), with a "likely range" of 2 to 4.5°C (about 3.6 to 8°F). But the report goes on to say that somewhat lower or "substantially higher" values cannot be ruled out.

On the regional scale, there is even more uncertainty.

How climate change affects Earth’s systems

In my post on tipping elements, I described nine Earth systems that are especially vulnerable to climate change – for example, Arctic summer sea ice and the Greenland ice sheet. If Earth systems like these are pushed past their tipping points, the resulting changes would have catastrophic effects on ecosystems and people. The latest IPCC report (WG2) devotes an entire chapter to this issue (Chapter 19, "Assessing key vulnerabilities and the risk from climate change").

But even if we knew exactly how much temperatures were going to rise, we could not say with certainty what the impact would be on Earth’s systems. We don’t know the precise temperature that will trigger severe or irreversible changes. Scientists’ best estimates, like those for greenhouse gas levels and temperatures, come in ranges.

For example, various studies estimate that Greenland could begin an irreversible meltdown once global average temperature rises 2.2 to 7.0°F above today’s temperature. With the warming in the pipeline, we’re just 1.2°F away from the lower end of that danger zone.

That’s already too close for comfort, but it may be even worse; scientists may have overestimated the resilience of ice sheets and other tipping elements to increasing temperature. Arctic summer sea ice is melting faster than predicted. Maybe this rapid loss is natural, random, and temporary, or maybe we’re closer to a tipping point than scientists thought. We don’t know for sure.

Adding up the uncertainties

The estimates of what we care about most – the impact on Earth’s systems – are based upon factors that are themselves estimates and subject to uncertainty. These uncertainties combine, so we end up with a best estimate surrounded by a wide range of uncertainty.

That’s why, even if nations could agree on which dangerous changes they want to avoid, scientists could not pinpoint the precise emissions levels that would guarantee that outcome.

Still, there are some things we know for sure. Scientists have no doubt that human-induced global warming is happening, and that the consequences, unchecked, are very dangerous. The uncertainty is only around exactly when and how it will play out.

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