Comments on the GWPF climate sensitivity report

climate sensitivity, GCMs, IPCC AR5, observations, projections, temperature, uncertainty

Guest post by Piers Forster, with comments from Jonathan Gregory & Ed Hawkins

Lewis & Crok have circulated a report, published by the Global Warming Policy Foundation (GWPF), criticising the assessment of equilibrium climate sensitivity (ECS) and transient climate response (TCR) in both the AR4 and AR5 IPCC assessment reports.

Climate sensitivity remains an uncertain quantity. Nevertheless, employing the best estimates suggested by Lewis & Crok, further and significant warming is still expected out to 2100, to around 3°C above pre-industrial climate, if we continue along a business-as-usual emissions scenario (RCP 8.5), with continued warming thereafter. However, there is evidence that the methods used by Lewis & Crok result in an underestimate of projected warming. Continue reading Comments on the GWPF climate sensitivity report →

The cascade of uncertainty in climate projections

projections, temperature, uncertainty, variability, visualisation

Climate projections have demonstrated the need to adapt to a changing climate, but have been less helpful (so far) in guiding how to effectively adapt. Part of the reason is the ‘cascade of uncertainty’ going from assumptions about future global emissions of greenhouse gases to what that means for the climate to real decisions on a local scale. Each of the steps in the process contains uncertainty, but which step is the most important? And, how might this be visualised? Continue reading The cascade of uncertainty in climate projections →

Atlantic overturning in decline?

Atlantic, MOC, observations, variability

Recent direct observations of the strength of the Atlantic Meridional Overturning Circulation (AMOC) show a decline of 10-15% since 2004. Is this a temporary fluctuation or part of a longer-term decline? A new analysis suggests that we might expect a further decline in the AMOC over the coming decade, which could impact on the climate of Europe and beyond. Continue reading Atlantic overturning in decline? →

Updates to comparison of CMIP5 models & observations

IPCC AR5, projections, temperature

As 2013 is nearly over, it is time for a short update to the comparisons of CMIP5 models and observations for global mean surface air temperatures. Part of the motivation for an update is the Cowtan & Way paper on spatial coverage biases in HadCRUT4, which has been given prominent attention in blogs and the media, notably the front page of The Independent. Continue reading Updates to comparison of CMIP5 models & observations →

Effects of recent observed vs RCP forcings

GCMs, projections, temperature

The recent global temperature hiatus has been explained by the IPCC AR5 as partly due to natural radiative forcings (solar & volcanic effects) and internal variability. Recently, other effects such as CFCs and biases in the observational coverage have also been suggested, as well as continuing uncertainty about the regional effects of aerosol forcings. When comparing simulations and observations, the CMIP5 simulations tend to use projected forcings rather than observed forcings after 2005. But what effect does this have? Continue reading Effects of recent observed vs RCP forcings →

Sources of uncertainty in CMIP5 projections

GCMs, IPCC AR5, projections, temperature, uncertainty, variability, visualisation

The recent IPCC AR5 includes a discussion on the sources of uncertainty in climate projections (Fig. 11.8, section 11.3.1.1), which updates previous analyses using CMIP3 (temperature, precipitation) to the latest CMIP5 simulations. The dominant source of uncertainty depends on lead time, variable and spatial scale. Continue reading Sources of uncertainty in CMIP5 projections →