Category Archives: GCMs

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

The slowdown zoo

Investigations into the recent observed slower rate of global warming have largely been focussed on variability in the Pacific basin. Climate models also show similar slowdowns focussed in the Pacific (e.g. Meehl et al. 2011).

But, is this the only type of simulated slowdown? How different can regional patterns of temperature change be for the same global change? Continue reading The slowdown zoo

Earth’s energy imbalance

Global surface air temperatures have risen less rapidly over the past 15 years than the previous few decades. The causes of this ‘hiatus’ have been much debated. However, just considering surface temperatures does not tell the whole story – a new analysis using satellite & ocean observations confirms that the Earth is still gaining energy overall. Continue reading Earth’s energy imbalance

Wet get drier (eventually)?

A prevailing paradigm of how rainfall patterns will change on a warming Earth is that the hydrological cycle strengthens causing wet regions to get wetter and dry regions to get drier.

However, this is not always the case: Hawkins, Joshi & Frame (2014) highlight one particular effect – the movement of the Inter-Tropical Convergence Zone (ITCZ) – as a key long-term driver of rainfall changes that do not follow this ‘wet get wetter’ paradigm. Continue reading Wet get drier (eventually)?

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

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

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

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

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

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

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

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

Reliability of regional climate trends

Climate information for the future is usually presented in the form of scenarios: plausible and consistent descriptions of future climate without probability information. This suffices for many purposes, but for the near term, say up to 2050, scenarios of emissions of greenhouse gases do not diverge much and we could work towards climate forecasts: calibrated probability distributions of the climate in the future. Continue reading Reliability of regional climate trends

Predicting changes in North Atlantic temperatures

The Earth is a complex system of interacting components, such as the atmosphere and ocean, which produce a wide variety of natural variability. This natural variability ensures that the evolution of a particular region’s climate, e.g. that of Western Europe, could be completely different to another region, or indeed the global mean climate. Such variability can impact on many areas of society; for example winter energy usage, or agriculture in sensitive regions. Continue reading Predicting changes in North Atlantic temperatures

Climate uncertainty: moving from ‘what’ to ‘when’

Update (23/10/11): The full article has now been published in Nature Climate Change

Climate projections (such as from the IPCC) usually consider the question of “what will happen to our future climate”. But, this question may be more informative if it is changed to “when will it happen”? Continue reading Climate uncertainty: moving from ‘what’ to ‘when’