Development of probability distributions for regional climate change from uncertain global mean warming and an uncertain scaling relationship

A method is presented to produce probability distributions for regional climate change in surface temperature and precipitation. The method combines a probability distribution for global mean temperature increase with the probability distributions for the appropriate scaling variables, i.e., the changes in regional temperature/precipitation per degree global mean warming. The distribution of each scaling variable is assumed to be normal. The uncertainty of the scaling relationship arises from systematic differences between the regional changes from global and regional climate model simulations and from natural variability. The contributions of these sources of uncertainty to the total variance of the scaling variable are estimated from simulated temperature and precipitation data in a suite of regional climate model experiments conducted in the framework of the EU funded project PRUDENCE, using an Analysis Of Variance (ANOVA). Five Case Study Regions (CSRs) are considered: NW England, the Rhine basin, Iberia, Jura lakes (Switzerland) and Mauvoisin dam (Switzerland). These CSRs cover the area considered in the EU funded project SWURVE. The resulting regional climate changes for 2070-2099 vary quite significantly between CSRs, between seasons, and between meteorological variables. A notable point is that for all CSRs the expected warming in summer is higher than the expected warming for the other seasons. This summer warming is accompanied by a large decrease in precipitation. The uncertainty of the scaling ratios for temperature and precipitation is relatively large in summer due to the differences between regional climate models. Differences between the spatial climate-change patterns of global climate model simulations significantly contribute to the uncertainty of the scaling ratio for temperature. However, for the scaling ratio for precipitation sometimes no meaningful contribution could be found due to the small number of global climate models in the PRUDENCE project and the large influence of natural variability. For precipitation, natural variability is often the largest source of uncertainty. By contrast, for temperature the contribution of natural variability to the total variance of the scaling ratio is small, in particular for the annual mean values. Simulation from the probability distributions of global mean warming and the scaling ratio results in a wider range of regional temperature change than that in the regional climate model experiments. For the regional change of precipitation, however, a large proportion of the simulations (about 90%) is within the range of the regional climate model simulations.

Regional climate model data used within the SWURVE project. 2: Addressing uncertainty in regional climate model data for five European case study areas

Abdelkader Mezghani

To aid assessments of climate change impacts on water related activities in the case study regions (CSRs) of the EC funded project SWURVE, estimates of uncertainty in climate model data need to be developed. In this paper, two methods to estimate uncertainty in annual surface temperature and precipitation for the period 2070-2099 are compared. Both methods combine probability distribution functions (PDFs) for global temperature increase and for scaling variables (i.e. the change in regional temperature/precipitation per degree of global annual average temperature change) to produce a probability distribution for regional temperature and precipitation. The methods differ in terms of the distribution used for the respective PDF. For scaling variables, the first method assumes a uniform distribution, whilst the second method assumes a normal distribution. For the PDF of global annual average temperature change, the first method uses a uniform distribution whilst the second uses a log-normal approximation to a distribution derived from Wigley and Raper, 2001. Although the methods give somewhat different ranges of change, they agree on how temperature and precipitation in each of the CSRs are likely to change relative to each other. For annual surface temperature, both methods predict increases in all CSRs, although somewhat less so for NW England (5th and 95th percentiles vary between 1.1-1.9 °C to 3.8-5.7 °C) and about 1.7-3.1 °C to 5.3-8.6 °C for the others. For precipitation, most probability distributions (except for NW England) show predominantly decreasing precipitation, particularly so for the Iberian CSR (5th and 95th percentiles vary from -29.3 - -44 % to -9.6 - -4 %).

2007

Evolution of stream and lake water temperature under climate change

Damien Bouffard, Wolf Hendrik Huwald, Adrien Michel, Carl Love Mikael Råman Vinnå, Bettina Schaefli, Martin Schmid, Alfred Johny Wüest

This report presents past observations and projects the future development of water temperature in Swiss lakes and rivers. Projections are made until the end of the 21st century using the CH2018 climate scenarios. Besides climate change effects on temperature, we also discuss effects on discharge for rivers, and effects on the thermal structure, and specifically the seasonal mixing regime and ice cover of lakes. Observations over the past 40 years show a clear increase in river temperatures, with a mean trend of 0.33 ± 0.03 °C per decade, corresponding to ~80% of the observed air temperature trend. This warming has been continuous over the last four decades and impacts the health of stream ecosystems (e.g. by favouring the spread of fish diseases) and their services (e.g., the water usage for industrial cooling). The temperature rise is more pronounced in the Swiss Plateau than in the Alps, where snow and glacier melt partially mitigates (for now) the effects of increasing air temperature. Conversely, annual average discharge shows no significant trend. Similar trends have also been reported for Swiss lakes with mean summer lake surface temperature increasing by 0.40 ± 0.08 °C per decade since the 1950s. This warming trend affects lake stratification. Warm periods may for instance increase the occurrence of deep-water anoxic conditions, as observed during the 2003 heat wave in Lake Zurich. In mild winters, ice cover duration is reduced in alpine lakes, and winter deep mixing is less intense in large-peri-alpine lakes. The mild winter 2006/7 limited, for instance, the seasonal mixing of Lake Constance to about 60 m depths. Effects of warming on lake thermal structure vary within and between regions, due to both lake and watershed characteristics as well as regional climate change patterns. We simulated the future evolution of stream temperature for 10 catchments in Switzerland for a historical reference period (1990–2000) and two future periods: 2055–2065 (mid-century) and 2080–2090 (end of the century). Results show that the temperature will stabilize by the end of the century for the RCP2.6 scenario (strong CO2 emission reduction), whereas the warming will accelerate with time for the RCP8.5 scenario (business as usual scenario). This expected warming will have significant impacts on the stream ecosystems. Alpine and lowland catchments will experience a similar annual mean temperature increase but display different seasonal effects. While Swiss Plateau rivers will become warmer both in winter and summer (but more in summer), alpine rivers will experience almost no warming in winter but a strong warming exceeding that of air temperature in summer. This is explained by an abrupt decrease in discharge, and by the soil warming resulting from the absence of snow and thus a lower albedo. Lake temperature projections are based on one-dimensional, vertically resolved, hydrodynamic simulations for 29 lakes. The simulated lakes cover a wide range of sizes, depths and water quality, and an altitude range from 200 to 1800 m a.s.l. Simulations indicate substantial changes in lake thermal structure for RCP8.5 with surface temperatures increasing on average by 3.3 °C at the end of the 21st century. This increase is limited to 0.9 °C in the mitigation scenario RCP2.6. We identified an altitude-dependent evolution of the durations of summer and winter stratification as well the ice-covered period. Larger changes in stratification duration are expected to occur at higher altitude lakes. Yet, these lakes will still maintain winter stratification and a shortened icecovered period while lower altitude lakes (below ~1500 m a.s.l.) risk drastic changes in the mixing regime. e.g., a complete loss of the ice cover and winter stratification under the RCP8.5 scenarios. Such changes in the mixing regime may strongly impact lake ecosystems. These low to mid altitude lakes may therefore be considered as the most vulnerable to climate change.

2021

Evolution of stream and lake water temperature under climate change

Damien Bouffard, Wolf Hendrik Huwald, Adrien Michel, Carl Love Mikael Råman Vinnå, Bettina Schaefli, Martin Schmid, Alfred Johny Wüest

2021