Immer wieder wird vor einer Verschlimmerung der globalen Dürren gewarnt. Gibt es wissenschaftliche Hinweise, die diese Warnungen untermauern? Samuel Shen und Kollegen veröffentlichten 2014 im Journal of the Atmospheric Sciences eine Analyse der globalen Niederschläge während der letzten drei Jahrzehnte. Dabei stießen sie auf einen Trend: Die Niederschläge haben sich mit einer Rate von o,o24 mm pro 100 Jahre erhöht. Das entspricht einer jährlichen Erhöhung von 0,00024 mm. Der Klimawandel schreitet voran, allerdings in wahnwitziger Ultrazeitlupe. Und nasser wird es auch. Rette sich wer kann! Hier der Abstract:
Multivariate Regression Reconstruction and Its Sampling Error for the Quasi-Global Annual Precipitation from 1900 to 2011
This paper provides a multivariate regression method to estimate the sampling errors of the annual quasi-global (75°S–75°N) precipitation reconstructed by an empirical orthogonal function (EOF) expansion. The Global Precipitation Climatology Project (GPCP) precipitation data from 1979 to 2008 are used to calculate the EOFs. The Global Historical Climatology Network (GHCN) gridded data (1900–2011) are used to calculate the regression coefficients for reconstructions. The sampling errors of the reconstruction are analyzed in detail for different EOF modes. The reconstructed time series of the global-average annual precipitation shows a 0.024 mm day−1 (100 yr)−1 trend, which is very close to the trend derived from the mean of 25 models of phase 5 of the Coupled Model Intercomparison Project. Reconstruction examples of 1983 El Niño precipitation and 1917 La Niña precipitation demonstrate that the El Niño and La Niña precipitation patterns are well reflected in the first two EOFs. Although the validation in the GPCP period shows remarkable skill at predicting oceanic precipitation from land stations, the error pattern analysis through comparison between reconstruction and GHCN suggests the critical importance of improving oceanic measurement of precipitation.
Ähnliches fanden Van Wijngaarden und Syed heraus, die ihre Ergebnisse 2015 im Journal of Hydrology publizierten. Die Regenfälle haben sich im globalen Durchschnitt in den letzten 150 Jahren nicht verändert. Abstract:
Changes in annual precipitation over the Earth’s land mass excluding Antarctica from the 18th century to 2013
Precipitation measurements made at nearly 1000 stations located in 114 countries were studied. Each station had at least 100 years of observations resulting in a dataset comprising over 1½ million monthly precipitation amounts. Data for some stations extend back to the 1700s although most of the data exist for the period after 1850. The total annual precipitation was found if all monthly data in a given year were present. The percentage annual precipitation change relative to 1961–90 was plotted for 6 continents; as well as for stations at different latitudes and those experiencing low, moderate and high annual precipitation totals. The trends for precipitation change together with their 95% confidence intervals were found for various periods of time. Most trends exhibited no clear precipitation change. The global changes in precipitation over the Earth’s land mass excluding Antarctica relative to 1961–90 were estimated to be: −1.2 ± 1.7, 2.6 ± 2.5 and −5.4 ± 8.1% per century for the periods 1850–2000, 1900–2000 and 1950–2000, respectively. A change of 1% per century corresponds to a precipitation change of 0.09 mm/year.
In den Highlights schreiben die Autoren:
Over 1½ million monthly precipitation totals observed at 1000 stations in 114 countries analysed.
Data record much longer than 3 recent conflicting studies that analysed a few decades of data.
No substantial difference found for stations located at northern, tropical and southern latitudes.
No substantial difference found for stations experiencing dry, moderate and wet climates.
No significant global precipitation change from 1850 to present.
Mit den Daten des Global Precipitation Climatology Project kann man eine Menge machen. Kailash Rajah und Kollegen nahmen 2014 in den Geophysical Research Letters regionale Unterschiede für die Zeit 1976-2000 unter die Lupe. Einige Kontinente wurden feuchter, andere trockener. Einen einheitlichen globalen Trend fanden die Forscher nicht. Hier der Abstract:
Changes to the temporal distribution of daily precipitation
Changes to the temporal distribution of daily precipitation were investigated using a data set of 12,513 land-based stations from the Global Historical Climatology Network. The distribution of precipitation was measured using the Gini index (which describes how uniformly precipitation is distributed throughout a year) and the annual number of wet days. The Mann-Kendall test and a regression analysis were used to assess the direction and rate of change to both indices. Over the period of 1976–2000, East Asia, Central America, and Brazil exhibited a decrease in the number of both wet and light precipitation days, and eastern Europe exhibited a decrease in the number of both wet and moderate precipitation days. In contrast, the U.S., southern South America, western Europe, and Australia exhibited an increase in the number of both wet and light precipitation days. Trends in both directions were field significant at the global scale.
Wie sieht es mit den Dürren aus. Sind sie häufiger geworden? Gregory McCabe und David Wolock haben es erforscht, im Juni 2015 in Earth and Space Science. Fazit: Die Dürre-Häufigkeit hat sich in den letzten 100 Jahren nicht verändert. Abstract:
Variability and trends in global drought
Monthly precipitation (P) and potential evapotranspiration (PET) from the CRUTS3.1 data set are used to compute monthly P minus PET (PMPE) for the land areas of the globe. The percent of the global land area with annual sums of PMPE less than zero are used as an index of global drought (%drought) for 1901 through 2009. Results indicate that for the past century %drought has not changed, even though global PET and temperature (T) have increased. Although annual global PET and T have increased, annual global P also has increased and has mitigated the effects of increased PET on %drought.
Im Januar 2015 dann eine interessante Arbeit von David Garcia-Garcia and Caroline Ummenhofer, wiederum in den Geophysical Research Letters. Die Wissenschaftler fanden etwas für Kalte-Sonne-Blogleser nicht ganz Unerwartetes heraus: Die Niederschläge der verschiedenen Regionen der variieren im Takt der Ozeanzyklen.Vielleicht wäre es an der Zeit, dies dann auch in die Klimamodelle so einzubauen. Abstract:
Multidecadal variability of the continental precipitation annual amplitude driven by AMO and ENSO
As the water vapor content in the atmosphere scales with temperature, a warmer world is expected to feature an intensification of the hydrological cycle. Work to date has mainly focused on mean precipitation changes, whose connection to climatic modes is elusive at a global scale. Here we show that continental precipitation annual amplitude, which represents the annual range between minimum and maximum (monthly) rainfall, covaries with a linear combination of the Atlantic Multidecadal Oscillation and low-frequency variations in the El Niño–Southern Oscillation on a decadal to multidecadal scale with a correlation coefficient of 0.92 (P < 0.01). The teleconnection is a result of changes in moisture transport in key regions. Reported trends in the annual amplitude of global precipitation in recent decades need to be assessed in light of this substantial low-frequency variability, which could mask or enhance an anthropogenic signal in hydrological cycle changes.
Auch Qiaohong Sun und Kollegen identifizierten die Ozeanzyklen als wichtigen Regentreiber, wie sie im Juni 2016 in den Geophysical Research Letters beschrieben:
Century-scale causal relationships between global dry/wet conditions and the state of the Pacific and Atlantic Oceans
The Granger causality test is used to examine the effects of the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the North Atlantic Oscillation (NAO) on global dry/wet conditions. The results show robust relationships between dry/wet conditions and the ocean states, as assessed through a multi-index (standardized precipitation evapotranspiration index and standardized precipitation index) and multiscale (3 months and 12 months) evaluation. The influence of ENSO events is widespread, dominating about 38% of the global land surface (excluding Antarctica). Southern and western North America, northern South America, and eastern Russia are influenced by the PDO. The NAO influences not only dry/wet conditions in Europe but also dry/wet conditions in northern Africa. Similarly, climate variability in southern Europe and northern Africa may be due to the concurrence of the ENSO and the NAO. Knowledge of the spatial influence of ocean states on global dry/wet conditions is valuable for improving drought and flood forecasting.
Nächste Studie. Diesmal von Jörg Steinkamp und Thomas Hickler, aus dem Jahr 2015 im Journal of Ecology. Ausgangspunkt war die Behauptung einiger Kollegen, dass die dürrebezogene Sterberate der globalen Wälder zugenommen habe. Die Autoren wühlten sich durch die Datenberge und kamen zu dem Schluss, dass man das nicht so pauschal behaupten könne. Zwar gab es in der Tat Dürreschäden bei den Wäldern, allerdings konnte kein global einheitlicher Trend festgestellt werden. Zudem waren Dürren wohl nur in einem Drittel aller Fälle der wahre Auslöser der Waldschäden. Abstract:
Is drought-induced forest dieback globally increasing?
Recently, it has been suggested that forest mortality has been generally increasing because of increasing drought and heat stress. But it is unclear if the observations at the investigated forest sites and regions are representative of forests globally and it has not been tested whether forest models are capable of reproducing these observations. We analysed historical climate data and used a dynamic global vegetation model (LPJ-GUESS) to assess (i) Which forests globally might have been affected by drought, (ii) If the field observations are representative for all forests and (iii) If the model can reproduce the reported mortality events.
Using two climate data sets and three drought indices, we identified no general global drying trend across all forests, but a large spatial variability. We neither detected a general increase in extreme drought events. A weak drying trend and an increase in extreme drought events were only apparent for forests in already dry climates and the locations or regions for which drought-induced mortality trends have been reported are predominantly in these dry climates.
LPJ-GUESS reproduced 66% of the reported mortality events and in 49% of the reported drought-induced mortality events drought was apparent in any of the climatologically derived drought indices. However, only in 30% of the cases simulated increased mortality coincided with drought events.
Synthesis. Our results indeed suggest that dry forests have been experiencing increasing drought-induced mortality. However, this does not apply to forests in general and the spatial variability has been large. The poor correspondence between the simulated and reported mortality events indicates that models like LPJ-GUESS driven by standard climatologies, and soil input data do not represent drought-induced mortality well. But the poor detection of the reported drought events in our climate indices also suggests that drought stress might not be the main driver of all the reported drought-mortality events.