A probability distribution for values of the effective climate sensitivity, with a lower bound of 1.6 K (5percentile), is obtained on the basis of the increase in ocean heat content in recent decades from analyses of observed interior ocean temperature changes, surface temperature changes measured since 1860, and estimates of anthopogenic and natural radiative forcing of the climate system. Radiative forcing is the greatest source of uncertainty in the calculation; the result also depends somewhat on the rate of ocean heat uptake in the late 19th century, for which an assumption is needed as there is no observational estimate. Because the method does not use the climate sensitivity simulated by a general circulation model, it provides an independent observationally based constraint on this important parameter of the climate system. where t is time and F is the heat flux into the ocean. Equation 1 has often been employed as the basis for energy-balance climate models. In the unperturbed steady-state climate, Q = F = 0 and ∆T = 0. If Q is raised from zero to some positive value, F becomes positive, additional heat is stored in the ocean, and ∆T rises. If Q then remains constant, F returns to zero over time, as the climate approaches a new steady state in which ∆T = Q/λ. From its definition, the equilibrium climate sensitivity ∆T 2× = Q 2× /λ, where Q 2× is the forcing that results from a doubling of the CO 2 concentration. Although it is defined in terms of a steady-state climate, the climate sensitivity can be estimated from any climate state. Provided we know F , Q and ∆T , we can calculate λ from Equation 1 and hence ∆T 2× (e.g. Cubasch et al., 2001). Some results with coupled atmosphere-ocean GCMs (AOGCMs) suggest that ∆T 2× (called the "effective" climate sensitivity when calculated from an unsteady climate) might not be constant even on the century timescale (Senior and Mitchell, 2000), although AOGCM experiments do not give rise to any expectation that it will change rapidly. If ∆T 2× is not constant, its usefulness for predicting future climate change is of course limited, and an estimate based on recent climate change is the most appropriate one to use. The utility of the climate sensitivity also depends on the response being independent of the nature of the agent causing the radiative forcing. 2 Method Recent studies aimed at setting constraints on the climate sensitivity have used climate models in which λ can be varied and heat uptake by the ocean is modelled simply (Wigley et al., 1997; Andronova and Schlesinger, 2001; Forest et al., 2002). The approach is systematically to adjust the parameters and inputs of the model, comparing the simulated results with observed surface temperature changes. The results give a range for ∆T 2× which is even wider than 1.5-4.5 • C. Using a model of ocean heat uptake inevitably involves assumptions about its mechanisms. Estimates of ocean heat uptake can instead be made using the five-year running means of observed ocean interior temperature changes of Levitus et al. (2...

This paper discusses a study of temperature and precipitation indices that may be suitable for the early detection of anthropogenic change in climatic extremes. Anthropogenic changes in daily minimum and maximum temperature and precipitation over land simulated with two different atmosphere-ocean general circulation models are analyzed. The use of data from two models helps to assess which changes might be robust between models. Indices are calculated that scan the transition from mean to extreme climate events within a year. Projected changes in temperature extremes are significantly different from changes in seasonal means over a large fraction (39%-66%) of model grid points. Therefore, the detection of changes in seasonal mean temperature cannot be substituted for the detection of changes in extremes. The estimated signal-to-noise ratio for changes in extreme temperature is nearly as large as for changes in mean temperature. Both models simulate extreme precipitation changes that are stronger than the corresponding changes in mean precipitation. Climate change patterns for precipitation are quite different between the models, but both models simulate stronger increases of precipitation for the wettest day of the year (4.1% and 8.8%, respectively, over land) than for annual mean precipitation (0% and 0.7%, respectively). A signal-to-noise analysis suggests that changes in moderately extreme precipitation should become more robustly detectable given model uncertainty than changes in mean precipitation.

The causes of twentieth century temperature change in six separate land areas of the Earth have been determined by carrying out a series of optimal detection analyses. The warming effects of increasing greenhouse gas concentrations have been detected in all the regions examined, including North America and Europe. In most regions, cooling from sulfate aerosols counteracts some of the greenhouse warming, and there is some evidence for reduced net aerosol cooling in Asia, possibly as a result of warming from black carbon.

No abstract

We analyse possible causes of twentieth century near-surface temperature change. We use an``optimal detection'' methodology to compare seasonal and annual data from the coupled atmosphere-ocean general circulation model HadCM2 with observations averaged over a range of spatial and temporal scales. The results indicate that the increases in temperature observed in the latter half of the century have been caused by warming from anthropogenic increases in greenhouse gases oset by cooling from tropospheric sulfate aerosols rather than natural variability, either internal or externally forced. We also ®nd that greenhouse gases are likely to have contributed signi®cantly to the warming in the ®rst half of the century. In addition, natural eects may have contributed to this warming. Assuming one particular reconstruction of total solar irradiance to be correct implies, when we take the seasonal cycle into account, that solar eects have contributed signi®cantly to the warming observed in the early part of the century, regardless of any relative error in the amplitudes of the anthropogenic forcings prescribed in the model. However, this is not the case with an alternative reconstruction of total solar irradiance, based more on the amplitude than the length of the solar cycle. We also ®nd evidence for volcanic in¯uences on twentieth century near-surface temperatures. The signature of the eruption of Mount Pinatubo is detected using annual-mean data. We also ®nd evidence for a volcanic in¯uence on warming in the ®rst half of the century associated with a reduction in mid-century volcanism.

No abstract

Editors note: For easy download the posted pdf of the Explaining Extreme Events of 2014 is a very low-resolution file. A high-resolution copy of the report is available by clicking here. Please be patient as it may take a few minutes for the high-resolution file to download.

Editors note: For easy download the posted pdf of the Explaining Extreme Events of 2016 is a very low-resolution file. A high-resolution copy of the report is available by clicking here. Please be patient as it may take a few minutes for the high-resolution file to download.