assembling quantitative understanding mapping precise futures calculating critical predictions delivering intelligent predictions assembling intelligent predictions mapping the future forecasting definitive forecasts calculating predictive contingencies delivering critical futures aggregating definitive predictions calculating contingent contingencies computing contingent wisdom computing critical wisdom predicting definitive futures


Metaculus Help: Spread the word

If you like Metaculus, tell your friends! Share this question via Facebook, Twitter, or Reddit.

What will the lower limit in the IPCC's ‘likely’ range of its climate sensitivity estimate be, in its 6th Assessment Report?

Climate sensitivity is arguably the most important number in climate change. It the the global-mean surface temperature change associated with a doubling of atmospheric CO2 concentration over its preindustrial value (Vial et al. 2013). The standard metric for climate sensitivity is the equilibrium climate sensitivity (ECS) given as the change in temperature at equilibrium for a doubling of CO2, neglecting long-term feedbacks associated with vegetation changes, carbon feedbacks and ice sheet dynamics (Skeie et al. 2017).

The first attempt to calculate climate sensitivity was in 1896, by the Swedish scientist, Svante Arrhenius. He found that CO2 doubling should increase the Earth's mean temperature by 5–6°C (Lapenis, 2006). Since then, hundreds of estimates have been made based on climate modelling and/or estimates based on physical evidence, such as climate changes in the distant past (Knutti et al., 2017).

Some have argued that from the 1980s onward, the uncertainty in climate projections has not narrowed appreciably, despite tremendous increases in computing power, in observations, and in the number of scientists studying the problem. For instance, In 2013, the IPCC widened the range of its estimate of climate sensitivity once again, opting for the "likely" range of 1.5°C to 4.5°C (66% confidence interval) (Freeman et al., 2015). The resolution of this dilemma has important implications for climate research and policy.

The ‘likely’ range (i.e. its 66% confidence interval) of ECS as stated in the IPCC's 5th annual report (IPCC, 2014) is 1.5–4.5 degrees Celsius. This is the range same that was givien in the IPCC's first report (IPCC, 1990). The IPCC's sixth report is currently scheduled for publication in 2022.

A 2018 article aims to revise the estimate of climate sensitivity, and reduces the range of possible end-of-century outcomes by more than half. It finds a central estimate of 2.8 degrees Celsius with 66 per cent confidence limits (equivalent to the IPCC ‘likely’ range) of 2.2–3.4 degrees Celsius.

Although seemingly beneficial, a reduction in the lower limit of the IPCC's lower limit in the range could be bad news. (Freeman et al.) argue that a lowering of the bottom of the range, although this reduces the mean of the estimate, our estimate of its standard deviation may increase. In turn, deeper uncertainty should magnify concerns, since marginal damages from rising temperatures increase rapidly.

What will the lower limit in the IPCC's ‘likely’ range of its climate sensitivity estimate be, in its sixth Assessment Report?


This question resolves as the lower of the 66% confidence limits, i.e. the lower number of its 'likely' range, of the IPCC's estimate for climate sensitivity in its sixth Assessment Report. The question resolves ambiguous if the sixth Assessment Report is not published before the end of 2024, or if it does not issue the estimate in the report.

In case of format changes to the IPCC's representation of their estimate for equilibrium climate sensitivity (i.e. different confidence interval is presented), an admin shall examine the relevant technical publication(s) produced by the IPCC, or those that principally inform the IPCC estimates for climate sensitivity, to work out the associated 66% confidence interval from their cumulative probability function (CDF). In case different confidence intervals are provided and the relevant CDF is not disclosed, we shall assume that the CDF is equivalent to the analytic form found in Roe and Baker (2007), so that the 66% confidence interval can be worked out.


Metaculus help: Predicting

Predictions are the heart of Metaculus. Predicting is how you contribute to the wisdom of the crowd, and how you earn points and build up your personal Metaculus track record.

The basics of predicting are very simple: move the slider to best match the likelihood of the outcome, and click predict. You can predict as often as you want, and you're encouraged to change your mind when new information becomes available.

The displayed score is split into current points and total points. Current points show how much your prediction is worth now, whereas total points show the combined worth of all of your predictions over the lifetime of the question. The scoring details are available on the FAQ.

Note: this question resolved before its original close time. All of your predictions came after the resolution, so you did not gain (or lose) any points for it.

Note: this question resolved before its original close time. You earned points up until the question resolution, but not afterwards.

This question is not yet open for predictions.

Thanks for predicting!

Your prediction has been recorded anonymously.

Want to track your predictions, earn points, and hone your forecasting skills? Create an account today!

Track your predictions
Continue exploring the site

Community Stats

Metaculus help: Community Stats

Use the community stats to get a better sense of the community consensus (or lack thereof) for this question. Sometimes people have wildly different ideas about the likely outcomes, and sometimes people are in close agreement. There are even times when the community seems very certain of uncertainty, like when everyone agrees that event is only 50% likely to happen.

When you make a prediction, check the community stats to see where you land. If your prediction is an outlier, might there be something you're overlooking that others have seen? Or do you have special insight that others are lacking? Either way, it might be a good idea to join the discussion in the comments.