Regime dependent changes in global precipitation

Assessment of changes in precipitation (P) as a function of percentiles of surface temperature (T) and 500 hPa vertical velocity (ω) are presented, considering present-day simulations and observational estimates from the Global Precipitation Climatology Project (GPCP) combined with the European Centre for Medium-range Weather Forecasts Interim reanalysis (ERA Interim). There is a tendency for models to overestimate P in the warm, subsiding regimes compared to GPCP, in some cases by more than 100%, while many models underestimate P in the moderate temperature regimes. Considering climate change projections between 1980–1999 and 2080–2099, responses in P are characterised by dP/dT ≥ 4%/K over the coldest 10–20% of land points and over warm, ascending ocean points while P declines over the warmest, descending regimes (dP/dT ∼ − 4%/K for model ensemble means). The reduced Walker circulation limits this contrasting dP/dT response in the tropical wet and dry regimes only marginally. Around 70% of the global surface area exhibits a consistent sign for dP/dT in at least 6 out of a 7-member model ensemble when considering P composites in terms of dynamic regime.

Current changes in tropical precipitation

Current changes in tropical precipitation from satellite data and climate models are assessed. Wet and dry regions of the tropics are defined as the highest 30% and lowest 70% of monthly precipitation values. Observed tropical ocean trends in the wet regime (1.8%/decade) and the dry regions (−2.6%/decade) according to the Global Precipitation Climatology Project (GPCP) over the period including Special Sensor Microwave Imager (SSM/I) data (1988–2008), where GPCP is believed to be more reliable, are of smaller magnitude than when including the entire time series (1979–2008) and closer to model simulations than previous comparisons. Analysing changes in extreme precipitation using daily data within the wet regions, an increase in the frequency of the heaviest 6% of events with warming for the SSM/I observations and model ensemble mean is identified. The SSM/I data indicate an increased frequency of the heaviest events with warming, several times larger than the expected Clausius–Clapeyron scaling and at the upper limit of the substantial range in responses in the model simulations.

Influence of dynamics on the changes in tropical cloud radiative forcing during the 1998 El Niño

Satellite measurements of the radiation budget and data from the U.S. National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis are used to investigate the links between anomalous cloud radiative forcing over the tropical west Pacific warm pool and the tropical dynamics and sea surface temperature (SST) distribution during 1998. The ratio, N, of the shortwave cloud forcing (SWCF) to longwave cloud forcing (LWCF) (N = −SWCF/LWCF) is used to infer information on cloud altitude. A higher than average N during 1998 appears to be related to two separate phenomena. First, dynamic regime-dependent changes explain high values of N (associated with low cloud altitude) for small magnitudes of SWCF and LWCF (low cloud fraction), which reflect the unusual occurrence of mean subsiding motion over the tropical west Pacific during 1998, associated with the anomalous SST distribution. Second, Tropics-wide long-term changes in the spatial-mean cloud forcing, independent of dynamic regime, explain the higher values of N during both 1998 and in 1994/95. The changes in dynamic regime and their anomalous structure in 1998 are well simulated by version HadAM3 of the Hadley Centre climate model, forced by the observed SSTs. However, the LWCF and SWCF are poorly simulated, as are the interannual changes in N. It is argued that improved representation of LWCF and SWCF and their dependence on dynamical forcing are required before the cloud feedbacks simulated by climate models can be trusted.

Simulation of late-twenty-first-century changes in wintertime atmospheric circulation over Europe due to anthropogenic causes

Evidence is presented, based on an ensemble of climate change scenarios performed with a global general circulation model of the atmosphere with high horizontal resolution over Europe, to suggest that the end-of-century anthropogenic climate change over the North Atlantic--European region strongly projects onto the positive phase of the North Atlantic Oscillation during wintertime. It is reflected in a doubling of the residence frequency of the climate system in the associated circulation regime, in agreement with the nonlinear climate perspective. The strong increase in the amplitude of the response, compared to coarse-resolution coupled model studies, suggests that improved model representation of regional climate is needed to achieve more reliable projections of anthropogenic climate change on European climate.

Late-Holocene changes in character and behaviour of land-terminating glaciers on James Ross Island, Antarctica

Virtually no information is available on the response of land-terminating Antarctic Peninsula glaciers to climate change on a centennial timescale. This paper analyses the topography, geomorphology and sedimentology of prominent moraines on James Ross Island, Antarctica, to determine geometric changes and to interpret glacier behaviour. The moraines are very likely due to a late-Holocene phase of advance and featured (1) shearing and thrusting within the snout, (2) shearing and deformation of basal sediment, (3) more supraglacial debris than at present and (4) short distances of sediment transport. Retreat of ∼100 m and thinning of 15–20 m has produced a loss of 0.1 km3 of ice. The pattern of surface lowering is asymmetric. These geometrical changes are suggested most simply to be due to a net negative mass balance caused by a drier climate. Comparisons of the moraines with the current glaciological surface structure of the glaciers permits speculation of a transition from a polythermal to a cold-based thermal regime. Small land-terminating glaciers in the northern Antarctic Peninsula region could be cooling despite a warming climate.

Monitoring present day changes in water vapour and the radiative energy balance using satellite data, reanalyses and models

A combination of satellite data, reanalysis products and climate models are combined to monitor changes in water vapour, clear-sky radiative cooling of the atmosphere and precipitation over the period 1979-2006. Climate models are able to simulate observed increases in column integrated water vapour (CWV) with surface temperature (Ts) over the ocean. Changes in the observing system lead to spurious variability in water vapour and clear-sky longwave radiation in reanalysis products. Nevertheless all products considered exhibit a robust increase in clear-sky longwave radiative cooling from the atmosphere to the surface; clear-sky longwave radiative cooling of the atmosphere is found to increase with Ts at the rate of ~4 Wm-2 K-1 over tropical ocean regions of mean descending vertical motion. Precipitation (P) is tightly coupled to atmospheric radiative cooling rates and this implies an increase in P with warming at a slower rate than the observed increases in CWV. Since convective precipitation depends on moisture convergence, the above implies enhanced precipitation over convective regions and reduced precipitation over convectively suppressed regimes. To quantify this response, observed and simulated changes in precipitation rate are analysed separately over regions of mean ascending and descending vertical motion over the tropics. The observed response is found to be substantially larger than the model simulations and climate change projections. It is currently not clear whether this is due to deficiencies in model parametrizations or errors in satellite retrievals.

Potential predictability of rapid changes in the Atlantic meridional overturning circulation

We explore the potential predictability of rapid changes in the Atlantic meridional overturning circulation (MOC) using a coupled global climate model (HadCM3). Rapid changes in the temperature and salinity of surface water in the Nordic Seas, and the flow of dense water through Denmark Strait, are found to be precursors to rapid changes in the model's MOC, with a lead time of around 10 years. The mechanism proposed to explain this potential predictability relies on the development of density anomalies in the Nordic Seas which propagate through Denmark Strait and along the deep western boundary current, affecting the overturning. These rapid changes in the MOC have significant, and widespread, climate impacts which are potentially predictable a few years ahead. Whilst the flow through Denmark Strait is too strong in HadCM3, the presence of such potential predictability motivates the monitoring of water properties in the Nordic Seas and Denmark Strait.

Climate change and heat-related mortality in six cities Part 2: climate model evaluation and projected impacts from changes in the mean and variability of temperature with climate change

Previous assessments of the impacts of climate change on heat-related mortality use the "delta method" to create temperature projection time series that are applied to temperature-mortality models to estimate future mortality impacts. The delta method means that climate model bias in the modelled present does not influence the temperature projection time series and impacts. However, the delta method assumes that climate change will result only in a change in the mean temperature but there is evidence that there will also be changes in the variability of temperature with climate change. The aim of this paper is to demonstrate the importance of considering changes in temperature variability with climate change in impacts assessments of future heat-related mortality. We investigate future heatrelated mortality impacts in six cities (Boston, Budapest, Dallas, Lisbon, London and Sydney) by applying temperature projections from the UK Meteorological Office HadCM3 climate model to the temperature-mortality models constructed and validated in Part 1. We investigate the impacts for four cases based on various combinations of mean and variability changes in temperature with climate change. The results demonstrate that higher mortality is attributed to increases in the mean and variability of temperature with climate change rather than with the change in mean temperature alone. This has implications for interpreting existing impacts estimates that have used the delta method. We present a novel method for the creation of temperature projection time series that includes changes in the mean and variability of temperature with climate change and is not influenced by climate model bias in the modelled present. The method should be useful for future impacts assessments. Few studies consider the implications that the limitations of the climate model may have on the heatrelated mortality impacts. Here, we demonstrate the importance of considering this by conducting an evaluation of the daily and extreme temperatures from HadCM3, which demonstrates that the estimates of future heat-related mortality for Dallas and Lisbon may be overestimated due to positive climate model bias. Likewise, estimates for Boston and London may be underestimated due to negative climate model bias. Finally, we briefly consider uncertainties in the impacts associated with greenhouse gas emissions and acclimatisation. The uncertainties in the mortality impacts due to different emissions scenarios of greenhouse gases in the future varied considerably by location. Allowing for acclimatisation to an extra 2°C in mean temperatures reduced future heat-related mortality by approximately half that of no acclimatisation in each city.

Projected impacts on heat-related mortality from changes in the mean and variability of temperature with climate change

The aim of this paper is to demonstrate the importance of changing temperature variability with climate change in assessments of future heat-related mortality. Previous studies have only considered changes in the mean temperature. Here we present estimates of heat-related mortality resulting from climate change for six cities: Boston, Budapest, Dallas, Lisbon, London and Sydney. They are based on climate change scenarios for the 2080s (2070-2099) and the temperature-mortality (t-m) models constructed and validated in Gosling et al. (2007). We propose a novel methodology for assessing the impacts of climate change on heat-related mortality that considers both changes in the mean and variability of the temperature distribution.

Developmental changes in presynaptic muscarinic modulation of excitatory and inhibitory neurotransmission in rat piriform cortex in vitro: relevance to epileptiform bursting susceptibility

Suppression of depolarizing postsynaptic potentials and isolated GABA-A receptor-mediated fast inhibitory postsynaptic potentials by the muscarinic acetylcholine receptor agonist, oxotremorine-M (10 microM), was investigated in adult and immature (P14-P30) rat piriform cortical (PC) slices using intracellular recording. Depolarizing postsynaptic potentials evoked by layers II-III stimulation underwent concentration-dependent inhibition in oxotremorine-M that was most likely presynaptic and M2 muscarinic acetylcholine receptor-mediated in immature, but M1-mediated in adult (P40-P80) slices; percentage inhibition was smaller in immature than in adult piriform cortex. In contrast, compared with adults, layer Ia-evoked depolarizing postsynaptic potentials in immature piriform cortex slices in oxotremorine-M, showed a prolonged multiphasic depolarization with superimposed fast transients and spikes, and an increased 'all-or-nothing' character. Isolated N-methyl-d-aspartate receptor-mediated layer Ia depolarizing postsynaptic potentials (although significantly larger in immature slices) were however, unaffected by oxotremorine-M, but blocked by dl-2-amino-5-phosphonovaleric acid. Fast inhibitory postsynaptic potentials evoked by layer Ib or layers II-III-fiber stimulation in immature slices were significantly smaller than in adults, despite similar estimated mean reversal potentials ( approximately -69 and -70 mV respectively). In oxotremorine-M, only layer Ib-fast inhibitory postsynaptic potentials were suppressed; suppression was again most likely presynaptic M2-mediated in immature slices, but M1-mediated in adults. The degree of fast inhibitory postsynaptic potential suppression was however, greater in immature than in adult piriform cortex. Our results demonstrate some important physiological and pharmacological differences between excitatory and inhibitory synaptic systems in adult and immature piriform cortex that could contribute toward the increased susceptibility of this region to muscarinic agonist-induced epileptiform activity in immature brain slices.

On the climate response of the low-latitude Pacific Ocean to changes in the global freshwater cycle

Under global warming, the predicted intensification of the global freshwater cycle will modify the net freshwater flux at the ocean surface. Since the freshwater flux maintains ocean salinity structures, changes to the density-driven ocean circulation are likely. A modified ocean circulation could further alter the climate, potentially allowing rapid changes, as seen in the past. The relevant feedback mechanisms and timescales are poorly understood in detail, however, especially at low latitudes where the effects of salinity are relatively subtle. In an attempt to resolve some of these outstanding issues, we present an investigation of the climate response of the low-latitude Pacific region to changes in freshwater forcing. Initiated from the present-day thermohaline structure, a control run of a coupled ocean-atmosphere general circulation model is compared with a perturbation run in which the net freshwater flux is prescribed to be zero over the ocean. Such an extreme experiment helps to elucidate the general adjustment mechanisms and their timescales. The atmospheric greenhouse gas concentrations are held constant, and we restrict our attention to the adjustment of the upper 1,000 m of the Pacific Ocean between 40°N and 40°S, over 100 years. In the perturbation run, changes to the surface buoyancy, near-surface vertical mixing and mixed-layer depth are established within 1 year. Subsequently, relative to the control run, the surface of the low-latitude Pacific Ocean in the perturbation run warms by an average of 0.6°C, and the interior cools by up to 1.1°C, after a few decades. This vertical re-arrangement of the ocean heat content is shown to be achieved by a gradual shutdown of the heat flux due to isopycnal (i.e. along surfaces of constant density) mixing, the vertical component of which is downwards at low latitudes. This heat transfer depends crucially upon the existence of density-compensating temperature and salinity gradients on isopycnal surfaces. The timescale of the thermal changes in the perturbation run is therefore set by the timescale for the decay of isopycnal salinity gradients in response to the eliminated freshwater forcing, which we demonstrate to be around 10-20 years. Such isopycnal heat flux changes may play a role in the response of the low-latitude climate to a future accelerated freshwater cycle. Specifically, the mechanism appears to represent a weak negative sea surface temperature feedback, which we speculate might partially shield from view the anthropogenically-forced global warming signal at low latitudes. Furthermore, since the surface freshwater flux is shown to play a role in determining the ocean's thermal structure, it follows that evaporation and/or precipitation biases in general circulation models are likely to cause sea surface temperature biases.

An Integrated Assessment of changes in the thermohaline circulation

This paper discusses the risks of a shutdown of the thermohaline circulation (THC) for the climate system, for ecosystems in and around the North Atlantic as well as for fisheries and agriculture by way of an Integrated Assessment. The climate model simulations are based on greenhouse gas scenarios for the 21st century and beyond. A shutdown of the THC, complete by 2150, is triggered if increased freshwater input from inland ice melt or enhanced runoff is assumed. The shutdown retards the greenhouse gas-induced atmospheric warming trend in the Northern Hemisphere, but does not lead to a persistent net cooling. Due to the simulated THC shutdown the sea level at the North Atlantic shores rises by up to 80 cm by 2150, in addition to the global sea level rise. This could potentially be a serious impact that requires expensive coastal protection measures. A reduction of marine net primary productivity is associated with the impacts of warming rather than a THC shutdown. Regional shifts in the currents in the Nordic Seas could strongly deteriorate survival chances for cod larvae and juveniles. This could lead to cod fisheries becoming unprofitable by the end of the 21st century. While regional socioeconomic impacts might be large, damages would be probably small in relation to the respective gross national products. Terrestrial ecosystem productivity is affected much more by the fertilization from the increasing CO2 concentration than by a THC shutdown. In addition, the level of warming in the 22nd to 24th century favours crop production in northern Europe a lot, no matter whether the THC shuts down or not. CO2 emissions corridors aimed at limiting the risk of a THC breakdown to 10% or less are narrow, requiring departure from business-as-usual in the next few decades. The uncertainty about THC risks is still high. This is seen in model analyses as well as in the experts’ views that were elicited. The overview of results presented here is the outcome of the Integrated Assessment project INTEGRATION.