Category Archives: uncertainty

Uncertainty in warming since pre-industrial times

The consequences of the Paris Agreement’s choice of the pre-industrial as its baseline have been discussed previously on this blog. This choice makes sense from a climate forcing perspective (as radiative forcings are measured with respect to a quasi-equilibrated state, and the well-observed recent past is not close to have finished responding to anthropogenic drivers). Looking back into the pre-industrial period, there are fewer instrumental observations of the temperature across the globe. So naturally our knowledge of the pre-industrial baseline temperature is uncertain.

Recent work, such as Hawkins et al. (2017) and Schurer et al. (2017), have looked to assess and quantify this uncertainty in light of future targets. The magnitude of this uncertainty, although small, becomes important when you consider the amount of warming left between today and the 1.5°C target. Continue reading Uncertainty in warming since pre-industrial times

Irreducible uncertainty in near-term climate projections

Uncertainty in climate projections arises from several different sources. For example, the future emissions scenario is not known, so the usual approach is to run several to compare plausible futures. In addition, each climate model produces a different change in climate. However, on regional spatial scales and for the next couple of decades, it is the internal variability of climate which dominates. These natural climate fluctuations provide an irreducible limit on the precision with which we can make predictions on such spatial and temporal scales – but how large is this limit? Continue reading Irreducible uncertainty in near-term climate projections

How not to use daily CMIP5 data for impact studies

A new paper out this week in PLOS Biology uses some CMIP5 simulations of daily mean surface air temperature as part of a larger analysis on the change to future plant growing days. The description of the analysis suggests they have not used the simulations appropriately to arrive at their conclusions. Here I highlight a couple of possible pitfalls in using such data in impact studies. Continue reading How not to use daily CMIP5 data for impact studies

Hiatus delays unprecedented warming rates

Current global temperatures are often discussed in terms of their unprecedented nature when compared to the last few thousand years. An interesting paper in Nature Climate Change by Steven J Smith and colleagues examines the rate of warming projected by the CMIP5 ensemble and suggests that the rate of warming is unprecedented also. However, we note here that their projections are not constrained by the current observations which do not show such strong warming rates at present, and are unlikely to do so in the next few years. Continue reading Hiatus delays unprecedented warming rates

Arctic sea-ice decline erratic as expected

Imagine a ball bouncing down a bumpy hill. Gravity will ensure that the ball will head downwards. But, if the ball hits a bump at a certain angle it might move horizontally or even upwards for a time, before resuming its inevitable downward trajectory. This bouncing ball is an analogy for the behaviour of Arctic sea-ice.

Post based on Swart et al., Nature Climate Change, or see a less technical summary. Continue reading Arctic sea-ice decline erratic as expected

Projected changes of precipitation and temperature extremes

Model projections of heavy precipitation and temperature extremes include large uncertainties. However, disagreement between individual simulations primarily arises from internal variability, whereas models agree remarkably well on the forced signal.

Post based on Fischer et al., 2014, Geophys. Res. Lett.
Continue reading Projected changes of precipitation and temperature extremes