Comparison of land surface humidity between observations and CMIP5 models

Abstract

Abstract.We compare the latest observational land surface humidity dataset, HadISDH, with the latest generation of climate models extracted from the CMIP5 archive and the ERA-Interim reanalysis over the period 1973 to present. The globally averaged behaviour of HadISDH and ERA-Interim are very similar in both humidity measures and air temperature, on decadal and interannual timescales.The global average relative humidity shows a gradual increase from 1973 to 2000, followed by a steep decline in recent years. The observed specific humidity shows a steady increase in the global average during the early period but in the later period it remains approximately constant. None of the CMIP5 models or experiments capture the observed behaviour of the relative or specific humidity over the entire study period. When using an atmosphere-only model, driven by observed sea surface temperatures and radiative forcing changes, the behaviour of regional average temperature and specific humidity are better captured, but there is little improvement in the relative humidity.Comparing the observed climatologies with those from historical model runs shows that the models are generally cooler everywhere, are drier and less saturated in the tropics and extra-tropics, and have comparable moisture levels but are more saturated in the high latitudes. The spatial pattern of linear trends is relatively similar between the models and HadISDH for temperature and specific humidity, but there are large differences for relative humidity, with less moistening shown in the models over the tropics and very little at high latitudes. The observed drying in mid-latitudes is present at a much lower magnitude in the CMIP5 models. Relationships between temperature and humidity anomalies (T -q and T -rh) show good agreement for specific humidity between models and observations, and between the models themselves, but much poorer for relative humidity. The T -q correlation from the models is more steeply positive than the observations in all regions, and this over-correlation may be due to missing processes in the models.The observed temporal behaviour appears to be a robust climate feature rather than observational error. It has been previously documented and is theoretically consistent with faster warming rates over land compared to oceans. Thus, the poor replication in the models, especially in the atmosphere-only model, leads to questions over future projections of impacts related to changes in surface relative humidity. It also precludes any formal detection and attribution assessment.

Recent Very Hot Summers in Northern Hemispheric Land Areas Measured by Wet Bulb Globe Temperature Will Be the Norm Within 20 Years

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Drivers of the enhanced decline of land near-surface relative humidity to abrupt 4xCO2 in CNRM-CM6-1

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Novel climates reverse carbon uptake of atmospherically dependent epiphytes: Climatic constraints on the iconic boreal forest lichen Evernia mesomorpha

“…Energy-added treatments represented realistic potential future warming (projected 5.9°C increase) expected for the SPRUCE site in the year 2085. The treatments had a less distinct match to potential changes in average relative humidity (projected 3 percentage-point increase), although average moisture changes are also expected to be accompanied by a larger range of variability at the northern Minnesota SPRUCE site based on comparison to forecast data (IPCC, 2014;Dunn et al, 2017). (A, B, C) are the estimated incremental effect of a 1°C increase in warming on the mean annual biomass growth rate, and shaded areas around lines are 95% confidence intervals of the estimated mean.…”

“…RH ( experiment was accompanied by proportional decreases in relative humidity as a product of the experimental design (Hanson et al, 2017), including a reduction in the potential for dew formation. Moisture reductions ranging from mild to drastic are projected for the Midwest region of the United States given plausible carbon emissions and atmospheric scenarios (Dunn et al, 2017), which could severely impact photosynthesis and biomass accumulation of poikilohydric epiphytes that rely on dew and humid air as sources of moisture instead of liquid rain (Lange et al, 2007;Gauslaa, 2014). Dew occurs at relative humidity approaching 100% and can thus be viewed as one point along an atmospheric moisture continuum (humid air < dew < liquid rain).…”