In #12 we saw that soil moisture droughts - agricultural and ecological droughts - have increased globally.
I’ve been following the flow of AR6 in their discussion of recent trends. They do go on to discuss hydrological droughts without much that’s definitive so perhaps we’ll have a brief look at that in another article, as I’m something of a completionist.
But there’s something important missing from the drought section.
Are plants dying? If not, is there really an increase in soil moisture drought?
Here’s a question from Alexis Berg & Justin Sheffield (2018) to put the problem in a broader context. Here and in all the other papers quoted, bold text is my change:
The notion that a warmer climate leads to a drier land surface, i.e., increased water stress, driven overwhelming by the effect of warmer temperatures on evaporative demand, appears, however, inconsistent with paleo-evidence and vegetation reconstructions for different colder and warmer past climates.
For instance, the Mid-Pliocene Warm Period, circa 3 Ma ago, is often considered a close enough analogue for current anthropogenic warming, because of similar continental configurations, land elevations and ocean bathymetries as today, and near-modern atmospheric CO2 in the range of 350– 450 ppm.
Climate model estimates indicate that the global annual mean surface temperature was 2.7–4.0 °C higher at that period than modern (pre-industrial) climate, with little low-latitude warming and strong polar amplification. However, vegetation reconstructions show a general increase in vegetation in this warmer world, including an expansion of tropical savannas and forests at the expense of deserts and a northwards shift of temperate and boreal vegetation zones.
Similarly, the Last Glacial Maximum (20 kyr ago), which was around 5°C cooler globally than modern climate, featured less forests and much wider distribution of dry vegetation biotopes, including deserts, grasslands, savanna, and dry steppe, suggesting greater hydrological constraint, on average, on ecosystems. While very low levels of atmospheric CO2 (190 ppm) at that time also likely limited vegetation productivity and contributed to these biogeographic differences, overall, such differences do not appear consistent with the notion that warmer (colder) climates are associated with substantially greater (lower) soil moisture stress on vegetation.
The apparent inconsistency between paleo-evidence and drought projections has been called the Aridity Paradox, with some authors arguing that it implies some level of misinterpretation of future drought/aridity projections from climate models.
They go to say:
It is also worth noting that over the last four decades, global greening, i.e., a widespread increase in leaf area index (LAI) measured by satellite, has been observed, including in semi-arid areas. Such an increase is qualitatively consistent with the large growth in gross primary productivity estimated over the twentieth century and would appear inconsistent with a concurrent increase in soil moisture deficits and vegetation water stress.
Here’s Xu Lian and co-authors in a Nature review paper from 2021. They are considering drylands - “Drylands describe regions subject to permanent or seasonal water deficiency, which currently occupy ~42% of global land surface”:
Although warming heightens vapour pressure deficit and, thus, atmospheric demand for water in both the observations and the projections, these changes do not wholly propagate to exacerbate soil moisture and runoff deficits. Moreover, counter-intuitively, many arid ecosystems have exhibited significant greening and enhanced vegetation productivity since the 1980s..
..A surge of research has, therefore, emerged to assess dryland changes in the past and predict their future trajectories, the findings of which are highly contradictory. For example, numerous studies suggest that global drylands have become more arid, yet others show the same regions experiencing greening and enhanced vegetation activity. This apparent inconsistency stems from different interpretations of aridity..
They also say:
Using microwave satellite observations of near- surface soil moisture, it is estimated that 38.4% of global drylands have experienced a significant drying trend since 1979, while only 2.9% showed a wetting trend.
However, other observation-driven datasets of total- column (or root-zone) soil moisture (such as GLEAM, TerraClimate and GLDAS) consistently reveal an increasing trend for the same period.
The divergence of soil moisture trends is likely because surface and deep-layer soil moistures are controlled by different processes — warming-induced rise in evaporative demand drives the reduction of surface soil moisture, whereas soil moisture in deep layers is more controlled by antecedent moisture status and vegetation activities. Weaker drying in the root zone compared with the surface soil suggests that deep-rooted plants in drylands are less likely to suffer severe soil moisture stress than shallow-rooted plants and crops.
They discuss the impact of more CO2 allowing plants to use water more efficiently.
In their conclusion:
This phenomenon [drier climate at the end of the last ice age, wetter climate in the warmer distant past] again contradicts with model-estimated lower Aridity Index values (land surface drying) for warmer periods, but does agree with present-day trends for increasing vegetation cover. That is, analysis from the current climate suggests that the physiological influence of higher CO2 levels in warmer periods shapes the surface water cycle and prevents the expansion of arid and semi-arid ecosystems, and this present-day process has presumably operated throughout the geological timescale.
I’ve included some extract from other papers in the Notes below for people interested.
Lastly, here’s Zaichun Zhu and co-authors from a Nature 2016 paper:
Yet how global vegetation is responding to the changing environment is not well established. Here we use three long-term satellite leaf area index (LAI) records and ten global ecosystem models to investigate four key drivers of LAI trends during 1982–2009.
We show a persistent and widespread increase of growing season integrated LAI (greening) over 25% to 50% of the global vegetated area, whereas less than 4% of the globe shows decreasing LAI (browning). Factorial simulations with multiple global ecosystem models suggest that CO2 fertilization effects explain 70% of the observed greening trend, followed by nitrogen deposition (9%), climate change (8%) and land cover change (LCC) (4%).
Chapter 11 on droughts doesn’t cover this topic at all. However, Working Group II, "Impacts, Adaptation & Vulnerability," does mention global greening. This part of the report is still a work in progress and may be subject to changes.
The droughts section in Chapter 11 seems incomplete without discussing this.
I’m 100 papers in so it’s easy to get a little lost.
The key points:
The average person reading that droughts have increased would expect plants are dying so it’s important to stress that the opposite seems to be the case
Are we seeing increased plant growth because the effect of more CO2 outweighs drying soil?
Is it actually true that soil moisture has decreased? Is it only surface moisture that has decreased but deeper soil has not? Has surface moisture actually decreased or is this an artifact of the calculation of rainfall minus evaporation, which has a lot of uncertainties?
Notes
A paper on semi-arid regions, Rasmus Fensholt et al 2012:
While many ground-based reports of declining vegetation productivity have been published over the last decades, a number of recent publications have shown a nuanced and, for some regions, positive picture. With this background, the paper provides an analysis of trends in vegetation greenness of semi-arid areas using AVHRR GIMMS from 1981 to 2007..
..It is concluded that semi-arid areas, across the globe, on average experience an increase in greenness. Further it is observed that increases in greenness are found both in semi-arid areas where precipitation is the dominating limiting factor for plant production and in semi-arid areas where air temperature is the primarily growth constraint.
Finally, in the analysis of changes in the intra-annual variation of greenness it is found that seemingly similar increases in greenness over the study period may have widely different explanations. This implies that current generalizations, claiming that land degradation is ongoing in semi-arid areas worldwide, are not supported by the satellite based analysis of vegetation greenness.
Here’s Michael Roderick and co-authors from 2015:
How will climatic aridity change in future as a consequence of the ongoing accumulation of greenhouse gases in the atmosphere? One can readily envisage that some regions will become more arid while some become less arid but what might be the overall trend in aridity?
.. Some earlier work concluded that aridity would generally increase because of increasing air temperature. That work was based on a model that assumed the change in potential evaporation to be solely determined by change in air temperature. With that assumption, an increase in air temperature will cause increased potential evaporation that inevitably leads to a calculated increase in aridity. While that temperature-based approach has been widely used, it has long been known to be based on incorrect physics..
.. It is now recognized that one should apply physically based approaches with potential evaporation calculated as a function of radiation, temperature, humidity, and wind speed.
.. The above-noted interpretation that ‘‘warmer is more arid’’ raises some very important scientific questions. First, two recent global analyses of trends in aridity since 1948 concluded that some regions have become more arid while others have become less arid but with little overall change in terms of global averages and little obvious trend in aridity within the global drylands.. The dichotomy between increasing potential evaporation deduced from calculations and the decrease from direct observations has yet to be resolved.
.. A second contradictory result is that independent satellite observations show a general greening over many parts of the global drylands since the 1980s. This greening trend has been, at least in part, attributed to the biological impact of elevated atmospheric [CO2] in dryland regions. That is important because the UNEP methodology used by Feng and Fu [2013] does not explicitly consider the direct biological impact of changes in atmospheric [CO2] on the water use efficiency of vegetation.
Here’s Milly & Dunne from Nature 2016:
By various measures (drought area and intensity, climatic aridity index , and climatic water deficits), some observational analyses have suggested that much of the Earth’s land has been drying during recent decades, but such drying seems inconsistent with observations of dryland greening and decreasing pan evaporation..
..A ubiquitous increase in estimates of potential evapotranspiration (PET), driven by atmospheric warming, underlies the drying trends, but may be a methodological artefact. Here we show that the PET estimator commonly used severely overpredicts the changes in non-water- stressed evapotranspiration computed in the climate models themselves in ACC experiments.
This overprediction is partially due to neglect of stomatal conductance reductions commonly induced by increasing atmospheric CO2 concentrations in climate models. Our findings imply that historical and future tendencies towards continental drying, as characterized by offline-computed runoff, as well as other PET-dependent metrics, may be considerably weaker and less extensive than previously thought.
As a counter-point, Wenping Yuan and co-authors from 2019:
Atmospheric vapor pressure deficit (VPD) is a critical variable in determining plant photosynthesis. Synthesis of four global climate datasets reveals a sharp increase of VPD after the late 1990s. In response, the vegetation greening trend indicated by a satellite-derived vegetation index (GIMMS3g), which was evident before the late 1990s, was subsequently stalled or reversed. Terrestrial gross primary production derived from two satellite-based models (revised EC-LUE and MODIS) exhibits persistent and widespread decreases after the late 1990s due to increased VPD, which offset the positive CO2 fertilization effect.
Here’s their graph, which doesn’t seem to match their text for one of the datasets:
Their discussion:
Our results support increased VPD [vapor pressure deficit] being part of the drivers of the widespread drought-related forest mortality over the past decades, which has been observed in multiple biomes and on all vegetated continents
References
Seneviratne et al, 2021: Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
Climate Change and Drought: the Soil Moisture Perspective, Alexis Berg & Justin Sheffield, Current Climate Change Reports (2018)
Multifaceted characteristics of dryland aridity changes in a warming world, Xu Lian et al, Nature Reviews - Earth & Environment (2021)
Greening of the Earth and its drivers, Zaichun Zhu et al, Nature Climate Change (2016)
Greenness in semi-arid areas across the globe 1981–2007 — an Earth Observing Satellite based analysis of trends and drivers, Rasmus Fensholt et al, Remote Sensing of Environment (2012)
On the assessment of aridity with changes in atmospheric CO2, Michael L. Roderick et al, Water Resources Research (2015)
Potential evapotranspiration and continental drying, P. C. D. Milly & K. A. Dunne, Nature Climate Change (2016)
Increased atmospheric vapor pressure deficit reduces global vegetation growth, Wenping Yuan et al, Science Advances (2019)
Thanks for digging up and presenting these conflicting hypotheses about drought. Awesome. One thing about climate change I was confident about was that aridity on land would increase. Now I'm not so sure. Perhaps the following thought experiment is useful:
Imagine a totally flat planet with no oceans. The total precipitable water vapor in the atmosphere is 2.5 cm and there is a much larger amount of water in the ground (to mimic the reservoir found in the ocean). Now imagine rising GHGs raise the temperature of this planet by 1 degC or some multiple thereof. The amount of water vapor in the atmosphere increases by 7%/K, but otherwise the planet can't become more arid because of conservation of matter: The water that evaporates must fall somewhere as precipitation! The predicted increase in transpiration ignores the fact that the evaporated water must come down somewhere. We understand this principle in rain forests, where enormous amounts of water evaporation from an enormous number of leaves. That evaporated moisture falls as rain in a thunderstorm later the same day.
However, in arid regions climate modelers may be wrongly assuming the water lost to transpiration "disappears". In arid regions where precipitation is infrequent or highly seasonal, the amount of water vapor in the air tomorrow depends on how much water vapor is brought in by prevailing winds or turbulent mixing. In California, where it never rains in the summer, the absolute amount of moisture in the air over the relatively cold ocean is relatively low and when that air warms over land, can become pleasantly or unpleasantly warm, but never sticky like elsewhere in the summer. The grass on the hills turns brown. However, we know there is still significant humidity in the air, because the danger of fire shoots up dramatically on rare days when really dry winds blow from the interior towards the coast. In a single day, those dry winds can suck "all" of the moisture out of the dry grass and fallen debris from trees, making it much more flammable. When those dry winds stop blowing, the danger goes away, presumably because the dry grasses and debris suck moisture from the Pacific out of the air without any rain falling????