Category Archives: GCMs

Top-of-atmosphere contribution to unforced variability in global temperature

GCMs, observations, temperature, uncertainty, variability

As the attention received by the ‘global warming hiatus’ demonstrates, global mean surface temperature (T) variability on decadal timescales is of great interest to both the general public and to scientists. Here, I will discuss a recently published paper (Brown et al., 2014) that attempts to contribute to this scientific discussion by investigating the impact of unforced (internal) changes in the earth’s top-of-atmosphere (TOA) energy budget on decadal T variability.

Guest post by Patrick Brown (Duke University) Continue reading Top-of-atmosphere contribution to unforced variability in global temperature →

Uncertainties in the timing of unprecedented climates

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

Back in October 2013, Nature published an analysis by Camilo Mora et al. which discussed when ‘unprecedented climates’ would emerge, with a focus on regions of high biodiversity.

The paper was highlighted by Nature with an associated News & Views article and received widespread media attention (e.g. Climate Central, National Geographic, Guardian, Grist, amongst many). The paper was also in the top 100 most discussed papers from 2013 according to Altmetric.

Unfortunately, it has since emerged that the analysis has some serious flaws. A ‘Brief Communication Arising’ (or Comment) has now been published by Hawkins et al. in Nature (freely available for one month), written by a large group which includes several IPCC Lead Authors, from both WG1 and WG2. There is also a ‘Reply’ from Mora et al., and a new News & Views (N&V) piece by Scott Power discussing the continuing disagreement between the author teams. This is the first ever N&V on a Comment in Nature.

This post provides a slightly less technical description of the issues with Mora et al.’s analysis. The errors in Mora et al.’s Reply are summarised in a separate post. The Carbon Brief blog has also produced some videos on the topic. Continue reading Uncertainties in the timing of unprecedented climates →

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 →

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 →

Time of emergence of a warming signal

emergence, GCMs, IPCC AR5, projections, temperature, uncertainty

The ‘signal’ of a warming climate is emerging against a background ‘noise’ of natural internal variability. Both the magnitude of the signal and the noise vary spatially and seasonally. As society and ecosystems tend to be somewhat adapted to natural variability, some of the impacts of any change will be felt when the signal becomes large relative to the noise. So, it is important to note where and when this might occur. Continue reading Time of emergence of a warming signal →

Near-term regional climate: the range of possibilities

communication, GCMs, temperature, uncertainty, variability, visualisation

What are the possible regional temperature trends over the coming few decades? Globally, on average, there is expected to be a long-term warming, but this is not necessarily true for any particular location or period. What are the probabilities of a local warming or cooling? Continue reading Near-term regional climate: the range of possibilities →