Dr Kevin Trenberth

Sunshine is delicious, rain is refreshing, wind braces us up, snow is exhilarating; there is really no such thing as bad weather, only different kinds of good weather.  ~John Ruskin

The global hydrological cycle and its changes over time are examined in light of observations and current understanding. A particular focus is on how precipitation changes as the climate changes and changes in extremes, including risk of flooding and drought.  Net changes in surface evaporation are fairly modest and a much larger percentage change occurs in the water-holding capacity as atmospheric temperatures increase (7% per ºC).  Where moisture supply is not limited (such as over oceans) a consequence is increased water vapor in the atmosphere which feeds storms and thus leads to more intense precipitation; increased water vapor, heavier rains and stronger storms are already observed to be happening. In these cases the Clausius-Clapeyron relationship leads to positive correlations between temperatures and precipitation.  However, the disparity between modestly enhanced evaporation and heavier rains means decreases in frequency of precipitation, longer dry spells and enhanced droughts. “It never rains but it pours!” Over land in summer and tropical continents, moisture is limited in supply and conditions tend to be either hot and dry or cool and wet. The diurnal cycle plays a strong role. In these cases temperature and precipitation are strongly negatively correlated.Generally, with more moisture, wet areas get wetter and dry areas get drier leading to the rich get richer and the poor get poorer syndrome.  However, with more precipitation per unit of upward motion in the atmosphere, i.e. “more bang for the buck”, the atmospheric circulation weakens, causing monsoons to falter.  In the tropics and subtropics, very strong patterns of precipitation denote convergence zones and monsoon troughs vs subtropical anticyclones and deserts, and changes are dominated by shifts in these patterns as sea surface temperatures change, with El Niño a good example. Dipole structures often result as one region becomes drier while another becomes wetter. The eruption of Mount Pinatubo in 1991 led to an unprecedented drop in land precipitation and runoff, and widespread drought as precipitation shifted over oceans and evaporation faltered, providing lessons for possible geoengineering.  It is important to understand not only changes in mean precipitation, but also the intensity, frequency, duration, and type, and this also applies to the storms that bring precipitation.  Understanding these profound consequences of climate change is especially important for water managers.

Dr Richard Allan

Climate models project an increase in global mean precipitation through this century with an increased frequency of the most intense, damaging rainfall events. Conversely subtropical regions experiencing drought are expected to extend in coverage. Powerful constraints upon future changes in the water cycle involve the robust dependence of water vapour, net atmospheric radiative cooling and surface evaporation on warming tendencies and there exists a sound physical basis for the expectation for wet portions of the tropics to become wetter at the expense of the already arid regions. In this talk observational evidence and current physical understanding of robust model responses in the water cycle will be reviewed.

Dr Oliver Wild

Long-term observations in different parts of the world indicate that the abundance of ozone in the troposphere is increasing. As ozone is a strong oxidant and a greenhouse gas, this has important implications for surface air quality, for the oxidative environment of the troposphere, and for climate. Reproducing these trends in chemistry-transport models has proved difficult, reflecting uncertainties in our understanding of emissions, chemical processing and important transport and mixing processes. While it is clear that increased anthropogenic emissions of ozone precursors are largely responsible for the observed changes in ozone, current models cannot represent the different regional trends in ozone, suggesting that natural dynamical processes (transport and stratosphere-troposphere exchange) and biospheric interactions (biogenic hydrocarbon emissions, biomass burning and land-use change) also have important roles to play. While a full assessment of the interactions between the biosphere, atmosphere and climate system awaits the upcoming generation of Earth System models capable of resolving these feedbacks, this talk describes recent studies exploring the strengths and weaknesses in our current understanding of how emissions and meteorological processes control tropospheric composition.

Dr Stephan Fueglistaler

The strong temperature dependence of water vapour pressure and the well defined temperature gradients of the mean temperature structure of the atmosphere yield a close relation between the general circulation and atmospheric moisture. This may be nowhere as evident as in the case of stratospheric water vapour. Apart from a well-understood chemical source (methane oxidation) and a minor sink primarily over the Antarctic vortex, water vapour entering the stratosphere is controlled in a relatively narrow layer around the tropopause. A handful of measurements of water vapour in the lower stratosphere over England, and knowledge of the latitudinal structure of tropopause temperatures allowed Brewer (1949) exactly 60 years ago to deduce the general circulation in the stratosphere (now bearing his name alongside that of Dobson). Brewer's deduction is even more remarkable in light of the fact that the theoretical underpinnings of the stratospheric circulations have not been established until several decades later. Hence, the allusion to the famous Rosetta stone - combining two knowns, here water vapour and temperature, to deduce the unknown, here the circulation, may be not all that far-fetched.

Brewer's deduction has stood the test of time, and more recent observations of stratospheric water vapour have revealed a most detailed picture of stratospheric transport (e.g. the 'tape recorder', Mote et al. 1996). Nevertheless, stratospheric water also has the aura of being an enigmatic tracer: (i) The partitioning between vapour and ice of water entering the stratosphere (and their associated processes) remains poorly quantified; and (ii) some observations suggest a long-term trend that is at odds with expectations based on tropical tropopause temperature trends. In light of the profound importance of the water vapour feedback for climate, the possibility of a case in the atmosphere where some combination of processes can override the tendency imposed by Clausius-Clapeyron requires careful analysis.

Here, I will show new observations of water isotopologues and model calculations of water vapour in the tropical tropopause layer. These new results may allow for the first time an accurate quantification of the partitioning between vapour and ice of water entering the stratosphere. An intriguing aspect of the results is that apparently the ice contribution is strongly tied to that from vapour. Only measurements of water isotopologues may be able to separate the two, but measurements over the period 1991-2007 do not indicate a substantial change in the contribution from ice. I will argue that the case of stratospheric water vapour is a simplified version of the corresponding problem in the troposphere, but that the conceptual insights from the simpler case may pave the way for improvements in our theoretical understanding of the water vapour feedback in the troposphere.

Dr Tim Palmer

An assessment of risk is the basis of rational decision making. The notion of risk combines probability with impact; here we are concerned with the probability of occurrence of some weather event, and the economic impact of such occurrence. Estimates of risk are discussed based on two examples from rather disparate timescales: weather and climate. Results will show that techniques to estimate risk on the weather timescale problem (where validation data exist) are in fact relevant for the climate-change problem (where, in general, validation data does not exist). More generally they support the notion of seamless prediction - using the insights and constraints of numerical weather prediction in climate-change prediction.

Professor Corinne Le Quere

The oceans absorb ~25% of the CO₂ emitted to the atmosphere by human activities every year. This CO₂ 'sink' occurs on top of a very active natural carbon cycle, involving chemical, physical, and biological processes. All the processes that regulate the natural carbon cycle are controlled by climate. Hence changes in climate (both natural and human-induced) are expected to affect the natural carbon cycle, with consequences for the oceanic sink of CO₂. Recent observations have shown that the CO₂ sinks of the Southern Ocean and North Atlantic have not increased at the rate expected from the buildup of CO₂ in the atmosphere, at least since the early 1980s.  The exact processes driving the observed changes are under debate, but they have been associated with surface warming, and changes in ocean circulation and biological activity, with important non-linear effects.  Changes in biological activity are particularly difficult to measure over large scale.  Recent efforts to gather relevant observations on vital rates and biomass distribution are starting to provide a global view of the distribution, importance, and vulnerability of marine ecosystems to climate change, and of their potential to feedback on climate through CO₂ and other gases. This presentation will review evidence of recent changes in the marine carbon cycle, and provide a global view of the importance of marine ecosystems for climate.