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> drought conditions are likely to increase due to warmer temperatures and shifting precipitation patterns Can anyone link me to resources that go into more de

> drought conditions are likely to increase due to warmer temperatures and shifting precipitation patterns Can anyone link me to resources that go into more detail about this? I feel like I keep coming across conflicting projections: 1) That a warmer climate is a wetter climate, but 2) There will be more droughts. I'm not trying to argue with either of these, I just want to understand what I'm missing — to try to fill in the gaps of understanding.

giraffe_lady 3y ago
I don't have a good single source on this but the main thing is inconsistency and variability. A growing season drought followed by a couple of unseasonable monsoons can be a wetter-than-average year by the numbers. That sort of thing.
arnsholt 3y ago
The mechanism, as I understand it, is that higher temperatures drives increased evaporation which leads to increased droughts. But the evaporated water in the atmosphere has to come back down again, which it will tend to do in a great big splash. These torrential rains, I’m led to believe, don’t really penetrate deep enough into the soil to alleviate the increased evaporation (too much too fast), and they also drove erosion which aggravates the droughts I think.
- barbazoo 3y ago
  You know more than me probably but another detail is that warm air can hold more water than cooler air.
- myshpa 3y ago
  You're right. > higher temperatures drives increased evaporation which leads to increased droughts Certain types of vegetation work better in higher temperatures. Remove vegetation altogether, and the combination of higher temperatures, wind, and rain will transform the land into deserts. Preserve old-growth forests, and they will remain lush, green, and wet regardless of temperature variations. > the evaporated water in the atmosphere has to come back down again, which it will tend to do in a great big splash Atmospheric rivers are interesting. https://en.wikipedia.org/wiki/Atmospheric_river https://en.wikipedia.org/wiki/Atmospheric_river > don’t really penetrate deep enough into the soil to alleviate the increased evaporation (too much too fast) That depends again largely on the type of vegetation growing there. Forests and wetlands can capture the rain and allow it to seep, while pastures and corn fields will retain very little, which can lead to soil erosion and droughts / water shortages.
- sidewndr46 3y ago
  It also can just come back down into the ocean, which doesn't really do anything useful if you're trying to grow crops.
barbazoo 3y ago
Not a single complete source but possibly a good pointer: https://archive.ipcc.ch/news_and_events/docs/SBSTA-44/Sbsta_drought_poster.pdf https://archive.ipcc.ch/news_and_events/docs/SBSTA-44/Sbsta_...
myshpa 3y ago
Attempt at ELI5: We cut down a tree, and not much changes (except for the sound, maybe :). We cut down a forest, and things still seem okay. We replace many forests with pastures, and everything appears fine. We prevent forests from regrowing, and everything continues to function as expected. However, if we continue this pattern for a long time, eventually the forests reach a breaking point. They lose their ability to retain water and generate new rain, causing them to dry out and cease sending moisture further inland. With even less rainfall in those areas, the forests dry up and die out, worsening the problem. https://phys.org/news/2022-08-global-forest-area-capita-decreased.html https://phys.org/news/2022-08-global-forest-area-capita-decr... Over the past 60 years, the global forest area has declined by 81.7 million hectares, a loss that contributed to the more than 60% decline in global forest area per capita. https://www.ucsusa.org/resources/whats-driving-deforestation https://www.ucsusa.org/resources/whats-driving-deforestation Just four commodities—beef, soy (for animal feed), palm oil, and wood products—drive the majority of tropical deforestation. https://en.wikipedia.org/wiki/Biotic_pump https://en.wikipedia.org/wiki/Biotic_pump
outside1234 3y ago
My understanding is that the climate will be overall warmer and wetter but that this will not be uniform and, in fact, the variance will be much higher (on both geospatial and temporal axes). So we can expect some areas to be much wetter and some areas to be much drier and that areas will have a higher temperature variance (colder in winter as winds from Arctic north are stronger to blow colder air deeper south etc.)
- ceejayoz 3y ago
  This, and some of those areas will be less likely to stay that way; flooding one year, drought the next. If it were consistently one way, it'd be easier to adjust.
Zamicol 3y ago
I'm curious as well, especially when considering humidity. Here in Colorado we've had two summers of historically high humidity. Last year humidity rolled in, the prairie greened, and then later the rains came. This year, the prairie has been green all summer thanks to rain and humidity. We've never had humidity like this so I never realized how important humidity was for plant growth. I had always given rain more weight than it deserved.
hnmullany 3y ago
There ARE conflicting projections. There are 2 schools - general aridification - which the CMIP6 projections kind of reflect, and the wet gets wetter, dry gets dryer schools. This paper does a good job of summarizing the issues: Zaitchik, B.F., Rodell, M., Biasutti, M. and Seneviratne, S.I., 2023. Wetting and drying trends under climate change. Nature Water, pp.1-12.
- hnmullany 3y ago
  Here's a summary of several papers taken from a paper I'm in the process of writing: Climate change is expected to alter global water regimes significantly, although there is disagreement on regional specifics and uncertainty in precipitation models (Trenberth et al., 2014). Precipitation and runoff modeling is complicated by incomplete historical field measurements, an inability to reproduce existing regional drying patterns, sensitivity to the choice of initial conditions for sea surface temperature, and poor aerosol forcing accuracy among other challenges (Dai, 2013). While these and other uncertainties, such as the net effects of vegetation feedback and CO2 levels on soil moisture are acknowledged by the authors, the most recent climate model ensemble, CMIP6, forecasts further whole year drying in western North America, Central America, Europe and the Mediterranean, the Amazon, Southern Africa, Southeast Asia and Australia (Cook et al., 2020). This general aridification forecast is not universally supported, and other researchers believe that since the paleoclimate record demonstrates that warmer paleoclimates are also wetter, it is more likely that the vegetation response modeling in CMIP6 is decisively incorrect, and that it is more likely that overall precipitation will increase while intra-year seasonal variability also increases under climate change (Zaitchik et al., 2023). The combination of increased temperatures, more variable soil moisture and higher CO2 levels make forecasting future crop yield responses under climate change challenging and response results from experimental simulation and statistical modeling vary widely (Jones et al., 2014). For example, in a well-controlled growth chamber study of wheat response under RCP8.5 temperature and CO2 levels, crop lifecycle shorted appreciably but yields were not affected and one of the study’s cultivars had approximately 25% higher yield under simulated climate change (Sabella et al. 2020). Conversely, a meta-analysis of 91 statistical studies of crop response showed a mildly negative response for most crops/climate combinations to climate, but the variance of results across the included studies was extremely high (Challinor et al. 2014). While future water regimes are uncertain, there is high certainty about how current water regimes compare to historical averages. With some regional exceptions, the last hundred and fifty years has been a well-watered period compared to the millennial average. From 951CE to 1500CE, the North American continent experienced four major multi-decadal droughts, and the early 19th century was a period of recurring severe drought in the America West (Cook et al., 2007). During the last millennium, Europe experienced two exceptional periods of multi-decadal dryness, from 1400 to 1480CE and from 1770 to 1840CE (the Sporer and Dalton minima respectively) (Ionita et al., 2021). More recently, Europe experienced significant agricultural drought in 2003, 2015 and 2018 and recent tree-ring research indicates that drought severity in 2015-2018 is anomalous compared to a baseline of 1600CE to present (Freund et al., 2023). Globally, drought frequency has increased in China and India; and recent droughts in the African Sahel are historically unprecedented in duration and frequency (Mishra and Singh, 2010). However, to date, the overall magnitude of the increase in drought severity, as measured by the Palmer Drought Severity Index (PDSI: range -10 to +10), has been small. In North America, for example, there has been a mean -0.3 decrease in the PDSI vs. a 1950-1970 baseline, compared to the historical overall range of approximately -4 to +4 (Trenberth et al., 2014).

giraffe_lady 3y ago

I don't have a good single source on this but the main thing is inconsistency and variability. A growing season drought followed by a couple of unseasonable monsoons can be a wetter-than-average year by the numbers. That sort of thing.

arnsholt 3y ago

The mechanism, as I understand it, is that higher temperatures drives increased evaporation which leads to increased droughts. But the evaporated water in the atmosphere has to come back down again, which it will tend to do in a great big splash. These torrential rains, I’m led to believe, don’t really penetrate deep enough into the soil to alleviate the increased evaporation (too much too fast), and they also drove erosion which aggravates the droughts I think.

barbazoo 3y ago
You know more than me probably but another detail is that warm air can hold more water than cooler air.
myshpa 3y ago
You're right. > higher temperatures drives increased evaporation which leads to increased droughts Certain types of vegetation work better in higher temperatures. Remove vegetation altogether, and the combination of higher temperatures, wind, and rain will transform the land into deserts. Preserve old-growth forests, and they will remain lush, green, and wet regardless of temperature variations. > the evaporated water in the atmosphere has to come back down again, which it will tend to do in a great big splash Atmospheric rivers are interesting. https://en.wikipedia.org/wiki/Atmospheric_river https://en.wikipedia.org/wiki/Atmospheric_river > don’t really penetrate deep enough into the soil to alleviate the increased evaporation (too much too fast) That depends again largely on the type of vegetation growing there. Forests and wetlands can capture the rain and allow it to seep, while pastures and corn fields will retain very little, which can lead to soil erosion and droughts / water shortages.
sidewndr46 3y ago
It also can just come back down into the ocean, which doesn't really do anything useful if you're trying to grow crops.

barbazoo 3y ago

Not a single complete source but possibly a good pointer: https://archive.ipcc.ch/news_and_events/docs/SBSTA-44/Sbsta_drought_poster.pdf https://archive.ipcc.ch/news_and_events/docs/SBSTA-44/Sbsta_...

myshpa 3y ago

Attempt at ELI5: We cut down a tree, and not much changes (except for the sound, maybe :). We cut down a forest, and things still seem okay. We replace many forests with pastures, and everything appears fine. We prevent forests from regrowing, and everything continues to function as expected. However, if we continue this pattern for a long time, eventually the forests reach a breaking point. They lose their ability to retain water and generate new rain, causing them to dry out and cease sending moisture further inland. With even less rainfall in those areas, the forests dry up and die out, worsening the problem. https://phys.org/news/2022-08-global-forest-area-capita-decreased.html https://phys.org/news/2022-08-global-forest-area-capita-decr... Over the past 60 years, the global forest area has declined by 81.7 million hectares, a loss that contributed to the more than 60% decline in global forest area per capita. https://www.ucsusa.org/resources/whats-driving-deforestation https://www.ucsusa.org/resources/whats-driving-deforestation Just four commodities—beef, soy (for animal feed), palm oil, and wood products—drive the majority of tropical deforestation. https://en.wikipedia.org/wiki/Biotic_pump https://en.wikipedia.org/wiki/Biotic_pump

outside1234 3y ago

My understanding is that the climate will be overall warmer and wetter but that this will not be uniform and, in fact, the variance will be much higher (on both geospatial and temporal axes). So we can expect some areas to be much wetter and some areas to be much drier and that areas will have a higher temperature variance (colder in winter as winds from Arctic north are stronger to blow colder air deeper south etc.)

ceejayoz 3y ago
This, and some of those areas will be less likely to stay that way; flooding one year, drought the next. If it were consistently one way, it'd be easier to adjust.

Zamicol 3y ago

I'm curious as well, especially when considering humidity. Here in Colorado we've had two summers of historically high humidity. Last year humidity rolled in, the prairie greened, and then later the rains came. This year, the prairie has been green all summer thanks to rain and humidity. We've never had humidity like this so I never realized how important humidity was for plant growth. I had always given rain more weight than it deserved.

hnmullany 3y ago

There ARE conflicting projections. There are 2 schools - general aridification - which the CMIP6 projections kind of reflect, and the wet gets wetter, dry gets dryer schools. This paper does a good job of summarizing the issues: Zaitchik, B.F., Rodell, M., Biasutti, M. and Seneviratne, S.I., 2023. Wetting and drying trends under climate change. Nature Water, pp.1-12.

hnmullany 3y ago
Here's a summary of several papers taken from a paper I'm in the process of writing: Climate change is expected to alter global water regimes significantly, although there is disagreement on regional specifics and uncertainty in precipitation models (Trenberth et al., 2014). Precipitation and runoff modeling is complicated by incomplete historical field measurements, an inability to reproduce existing regional drying patterns, sensitivity to the choice of initial conditions for sea surface temperature, and poor aerosol forcing accuracy among other challenges (Dai, 2013). While these and other uncertainties, such as the net effects of vegetation feedback and CO2 levels on soil moisture are acknowledged by the authors, the most recent climate model ensemble, CMIP6, forecasts further whole year drying in western North America, Central America, Europe and the Mediterranean, the Amazon, Southern Africa, Southeast Asia and Australia (Cook et al., 2020). This general aridification forecast is not universally supported, and other researchers believe that since the paleoclimate record demonstrates that warmer paleoclimates are also wetter, it is more likely that the vegetation response modeling in CMIP6 is decisively incorrect, and that it is more likely that overall precipitation will increase while intra-year seasonal variability also increases under climate change (Zaitchik et al., 2023). The combination of increased temperatures, more variable soil moisture and higher CO2 levels make forecasting future crop yield responses under climate change challenging and response results from experimental simulation and statistical modeling vary widely (Jones et al., 2014). For example, in a well-controlled growth chamber study of wheat response under RCP8.5 temperature and CO2 levels, crop lifecycle shorted appreciably but yields were not affected and one of the study’s cultivars had approximately 25% higher yield under simulated climate change (Sabella et al. 2020). Conversely, a meta-analysis of 91 statistical studies of crop response showed a mildly negative response for most crops/climate combinations to climate, but the variance of results across the included studies was extremely high (Challinor et al. 2014). While future water regimes are uncertain, there is high certainty about how current water regimes compare to historical averages. With some regional exceptions, the last hundred and fifty years has been a well-watered period compared to the millennial average. From 951CE to 1500CE, the North American continent experienced four major multi-decadal droughts, and the early 19th century was a period of recurring severe drought in the America West (Cook et al., 2007). During the last millennium, Europe experienced two exceptional periods of multi-decadal dryness, from 1400 to 1480CE and from 1770 to 1840CE (the Sporer and Dalton minima respectively) (Ionita et al., 2021). More recently, Europe experienced significant agricultural drought in 2003, 2015 and 2018 and recent tree-ring research indicates that drought severity in 2015-2018 is anomalous compared to a baseline of 1600CE to present (Freund et al., 2023). Globally, drought frequency has increased in China and India; and recent droughts in the African Sahel are historically unprecedented in duration and frequency (Mishra and Singh, 2010). However, to date, the overall magnitude of the increase in drought severity, as measured by the Palmer Drought Severity Index (PDSI: range -10 to +10), has been small. In North America, for example, there has been a mean -0.3 decrease in the PDSI vs. a 1950-1970 baseline, compared to the historical overall range of approximately -4 to +4 (Trenberth et al., 2014).