In climate research, computer models have been used for many years to simulate future climatic conditions on Earth. The future projections of these climate models are included, for example, in the reports of the Intergovernmental Panel on Climate Change, which are an important basis for political decisions. To assess the quality and reliability of climate models, they are evaluated against independent observations. Measurements from the GRACE and GRACE-FO gravity field missions are also used for this purpose.
Dr. Laura Jensen, GFZ / Prof. Dr. Annette Eicker, HCU Hamburg
Since 2002, a global map of the terrestrial water storage (TWS), i.e. the total amount of water in soil, groundwater, snow and surface waters, can be derived for each month from the GRACE/GRACE-FO satellite data. Evaluating all maps from the last 20 years reveals long-term TWS trends - areas where it has become drier or wetter (Fig. 1, left).
Such TWS trends can also be derived from climate model data, even over much longer time periods (from 1850 to 2100), with assumptions made about socioeconomic development and greenhouse gas emissions to calculate future values. However, TWS trends from climate models have relatively large uncertainties and can differ from model to model. Therefore, in a study of the comparison of model TWS trends with GRACE-/GRACE-FO observations, the evaluation was limited to those regions where the majority of models agree on the direction of the trend (Fig. 1, right).
© Laura Jensen, GFZ
By comparing these with the direction of the GRACE-/GRACE-FO trend, it was then possible to identify areas that are likely to be affected by drying or wetting in the long term (until 2100). For example, according to the study, the Mediterranean region will become increasingly dry, as will the western United States and Central America, while there are indications that Central Africa will become wetter (Fig. 2).
© Laura Jensen, GFZ
However, GRACE-/GRACE-FO trends in some areas are overlaid by non-hydrologic mass changes (e.g., glacial isostatic adjustment) that are not included in climate models. Such signals are reduced in the evaluation by means of geophysical models, but these are not perfect and therefore a residual signal may remain that makes comparison difficult. Climate modelsClimate models are computer-aided tools that simulate the climate system by solving the basic physical equations describing the climate system on supercomputers. The resulting computer models are usually used to analyse data, understand climate proce... also do not optimally account for all hydrological processes, which means that trends can be systematically distorted.
In addition, long-term trends are often overlaid by short-term (interannual) natural climate variations. For example, due to phenomena such as El Niño or La Niña, an area that dries out over the long term may be unusually wet for several years. If this falls just within the observation period - which is relatively short for climate applications - the long-term drying may not yet be detected. It is estimated that at least 30 years of observations are needed before long-term trends and short-term variations can be well separated globally. However, studies also show that the influence of short-term variations is not large everywhere, so comparison with the 20-year GRACE-/GRACE-FO time series today already provides valuable hints to "hot spots" of climate-driven drying or wetting. Also, identifying areas where models and observations contradict each other can be an important starting point for further development of climate models, where GRACE-/GRACE-FO data can contribute.
References
- Jensen, L., A. Eicker, H. Dobslaw, T. Stacke, and V. Humphrey. “Long-Term Wetting and Drying Trends in Land Water Storage Derived From GRACE and CMIP5 Models.” Journal of Geophysical Research: Atmospheres 124, no. 17–18 (2019): 9808–23. doi.org/10.1029/2018JD029989.
- Jensen, L. “Satellite Gravimetry for Climate Model Evaluation.” Doctoral Thesis, HafenCity University Hamburg, 2021. doi.org/10.34712/142.23.