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Study reveals long-range levers behind U.S. southwest drought and drought future

Study reveals long-range levers behind U.S. southwest drought and drought future

The American Southwest has suffered a historic drought over the past two decades. A new study sheds light on the drivers and says things will never return to the relatively wet 20th century. (Kevin Krajik/Earth Institute)

The U.S. Southwest has been suffering from a drought that began in the late 20th century, and now it’s official known as the Great Drought, due to its strength and longevity.climate scientist Richard Seeger Been working on it since the beginning.Most of his research is on Natural and human-caused climate change.

in a New research in the journal NPJ Climate and Atmospheric SciencesSeager and his colleagues at Columbia University Lamont-Doherty Earth Observatory Link much of the drying to natural atmospheric circulation in the Pacific and Atlantic Oceans. They modeled these future cycles, saying that in the best-case scenario, conditions may ease in the coming years, but with climate change, there will never be a return to pre-2000 conditions. In the worst-case scenario, droughts will become more severe and persistent. We spoke with Seager about the study and what led to it.

What happened in the American Southwest? Has the area ever seen anything like this?
The Southwest has not experienced a drought of this magnitude since European settlement, although we know from tree rings that an equally severe event occurred earlier in the last millennium. However, this can happen when a region’s population and resulting water demand surges. These include the needs of agriculture, which is undoubtedly the main user. So the challenges are already huge. The question now is how this situation will evolve in the coming decades as climate change intensifies.

What are the natural mechanisms at work here?
Using climate models, we found that megadroughts, defined as summer soil moisture, were caused by reduced precipitation in the cool December-May season. This decline is caused by decadal-scale changes in tropical Pacific sea surface temperatures. This so-called Pacific Decadal Oscillation (PDO) is considered a natural oscillation. late 20th centuryth At the beginning of the century, it was in a relatively warm phase in the eastern seas. This affects atmospheric circulation, bringing a wet winter and spring to the Southwest. After the 1997-98 El Niño, the PDO shifted into a cold phase; this resulted in changes in atmospheric circulation, resulting in dry winters and springs that maintained humidity into summer.

Many of your colleagues say that climate warming is the main cause of droughts because rising temperatures are the main factor in absorbing soil moisture. Are they right, wrong, or somewhere in between?
They are almost certainly right. However, they focus on changes that have occurred over the past century. In our study, we focused on dramatic changes during the wet decades of the late 20th A major drought in the 21st centuryYingshi century. This change is driven almost entirely by reduced cold season precipitation.

Will the extreme drought continue?
We use a number of novel climate models to understand how conditions might evolve in 2040, influenced by potential changes in Pacific and Atlantic ocean temperatures. The best-case scenario is for the tropical Pacific to return to warmth and the North Atlantic to enter one of the cooler phases.However, even in this scenario, water availability will not return to what it was at the end of the 20th century, as climate change affects precipitation and warms the surface of the century level. In the worst-case scenario, the tropical Pacific continues to cool and the North Atlantic warms, and megadroughts will continue or even intensify.

There’s been a lot of talk lately about the fact that global warming is expected to cause warming in the tropical Pacific, but as you point out, we’re seeing the opposite. Does this mean the models we use to predict the future are broken?
To understand this, we created two sets of future forecasts. One is that natural decadal variability in sea surface temperatures is due to a background trend of warming in the eastern tropical Pacific, as shown by state-of-the-art climate models. In the other case, we extrapolate the actual trend. As I said, when the eastern tropical Pacific is cooler, the Southwest tends to get less cold season precipitation. So if things continue, both the best and worst-case scenarios will be drier than state-of-the-art climate models predict. We argue that these models indicate that the eastern tropical Pacific will warm due to long-term model biases, whereas in the real world the tropical Pacific responds to rising greenhouse gases by warming in large parts but not in the east. So, yes, the models are likely underestimating how dry it will be. Therefore, we believe that the worst case scenario may be the most realistic scenario.

How will overall warming affect future conditions?
Although we believe that ocean decadal variability has a larger impact in the Southwest, we believe that overall ocean warming will also reduce cold season precipitation in the Southwest. In addition, global warming is causing surface temperatures to rise in the Southwest. This allows the atmosphere to hold more water, thereby reducing soil moisture. According to our projections, the drying effects of this warming will become evident over the next two decades. This happens regardless of the pattern of sea surface temperature changes.

Will we see changes in precipitation in other parts of the world?
There will definitely be! We intend to use these same model experiments to study how decadal variability in the ocean and greenhouse gas-driven sea surface temperature changes will affect precipitation in other regions. Additionally, we are currently studying how the continued lack of warming in the eastern equatorial Pacific will affect tropical cyclone trends in the Pacific and Atlantic basins. We also need to focus on other parts of the world where the tropical Pacific has a strong influence, including South America, Africa and Asia.

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