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Thousands of years ago, a warm Arctic made mid-latitudes drier - Paleoclimate records show precipitation trends that could be repeated.


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2019 Mar 29, 12:05pm   471 views  3 comments

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The thing about a global climate change is that it isn’t as simple as shifting the temperatures everywhere by a set number of degrees. The temperature change isn’t uniform around the globe, and these regional differences can drive considerable knock-on effects on weather patterns.

The Arctic, for example, will warm more than the equatorial region. For our current global warming venture, there will be consequences of this fact beyond the Arctic itself. One juicy hypothesis is that the greater Arctic warming affects the behavior of the polar jet stream, driving significant changes on extreme weather patterns in the mid-latitudes. This idea is the subject of ongoing research, as well as genuine scientific debate and uncertainty.


The Arctic is the fastest-warming region of the planet.

Lessons from the past

One way to study patterns like this is to look to past climate changes. That’s what a team led by Northern Arizona University’s Cody Routson did, compiling paleoclimate records of rainfall in the Northern Hemisphere over the last 10,000 years.

This spans most of the period known as the Holocene—the warmer “interglacial” that has followed the end of the last ice age. The clockwork timing of the ice ages has been driven by cyclical wobbles in Earth’s orbit, which slightly alter the strength of summer sunlight in the high-latitude north—where most of the great ice sheets were located. That summer sunlight cycle peaked around 10,000 years ago, and Arctic temperatures started to slowly decline shortly after that.

That means that the trend from 7,000 years or so up to the Industrial Revolution was the polar opposite (if you will) of our current human-caused warming trend: as the sensitive Arctic cooled more quickly than the equator, the pole-to-equator temperature difference grew. The question the researchers set out to answer was how this changing pole-to-equator temperature difference affected the amount of precipitation in the mid-latitudes.

To answer, they compiled as many published temperature and precipitation records as possible for the last 10,000 years, based on everything from tree rings to insects found in lake mud. These records were separated into latitude bands from the equator to the Arctic. Based on the temperature data, the researchers were also able to calculate the pole-to-equator temperature difference as it grew over time.

In the mid-latitudes (between 30° and 50° north of the equator), the early part of this time period was actually significantly drier than today, with precipitation increasing over the millennia that followed.


Over the last 10,000 years, the temperature difference between the Arctic and the equator got larger (down on this graph), and precipitation increased.

Turning to models

To investigate how these two things were related, the team turned to climate model simulations of the last 10,000 years. The simulated pole-to-equator temperature difference matched the paleoclimate records pretty nicely, and the same pattern of increasing precipitation showed up, too, though the increase appeared to be smaller in the models.

The mid-latitude precipitation increase turned out to be driven by atmospheric circulation changes. In the drier early time period, the weaker pole-to-equator temperature difference made for a weaker jet stream and westerly winds and weaker versions of the spinning storm systems that typically account for the biggest rainstorms in the mid-latitudes. Running up to recent centuries, all those things strengthened, leading to more precipitation.


Conceptual diagram of the basic processes in play.

Projecting the future is not as simple as running these simulations in reverse, with a rapidly warming Arctic weakening winds and reducing rainfall in the mid-latitudes. The climate patterns of the Holocene were driven by influence of orbital cycles on sunlight, whereas humans are warming Earth’s climate in a different way. And while the researchers looked at the mid-latitude average here, there could be important regional differences that are hard to see with local paleoclimate records from a limited number of spots. Still, the physical process they’re identifying is something that can be studied more closely to understand how it will behave in the future.

“Currently,” the researchers write, “the northern high latitudes are warming at rates nearly double the global average, decreasing the Equator-to-pole temperature gradient to values comparable with those in the early to middle Holocene. If the patterns observed during the Holocene hold for current anthropogenically forced warming, the weaker latitudinal temperature gradient will lead to considerable reductions in mid-latitude water resources.”

https://arstechnica.com/science/2019/03/thousands-of-years-ago-a-warm-arctic-made-mid-latitudes-drier/?comments=1

Note: The comments section on this article is worth reading - intelligence coupled with stupid

#Science #Climate #Weather

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1   anonymous   2019 Mar 30, 3:09am  

How frigid polar vortex blasts are connected to global warming

In the past several years, thanks to previous cold waves, the polar vortex has become entrenched in our everyday vocabulary and served as a butt of jokes for late-night TV hosts and politicians. But what is it really? Is it escaping from its usual Arctic haunts more often? And a question that looms large in my work: How does global warming fit into the story?

Rivers of air

Actually, there are two polar vortices in the Northern Hemisphere, stacked on top of each other. The lower one is usually and more accurately called the jet stream. It’s a meandering river of strong westerly winds around the Northern Hemisphere, about seven miles above Earth’s surface, near the height where jets fly.

The jet stream exists all year, and is responsible for creating and steering the high- and low-pressure systems that bring us our day-to-day weather: storms and blue skies, warm and cold spells. Way above the jet stream, around 30 miles above the Earth, is the stratospheric polar vortex. This river of wind also rings the North Pole, but only forms during winter, and is usually fairly circular.



Both of these wind features exist because of the large temperature difference between the cold Arctic and warmer areas farther south, known as the mid-latitudes. Uneven heating creates pressure differences, and air flows from high-pressure to low-pressure areas, creating winds. The spinning Earth then turns winds to the right in the northern hemisphere, creating these belts of westerlies.

Why cold air plunges south

Full Article: http://theconversation.com/how-frigid-polar-vortex-blasts-are-connected-to-global-warming-110653
2   anonymous   2019 Mar 30, 3:09am  

Not sure how many people saw this in January of this year...the blue is the cold - notice how warm most of the planet was during that period.


Predicted daily mean, near-surface temperature (C) differences from normal (relative to 1979-2000) for Jan. 28-30, 2019. Data from NOAA’s Global Forecast System model. Climate Reanalyzer, Climate Change Institute, University of Maine.,
3   anonymous   2019 Apr 1, 6:17am  

Hurricanes to deliver a bigger punch to coasts

Scientists are working to improve their forecasts for hurricane winds and waves, and research on ocean and atmosphere interactions is boosting our understanding of the relationship between climate and the formation of hurricanes. Still, there is considerable uncertainty in predicting trends in extreme weather conditions 100 years into the future. Some computer simulations suggest possible changes in these storms due to climate change.

For example, scientists have computed detailed simulations of hurricane-type storms for future climate-warming scenarios and revealed that in some cases the hurricane season could be longer. The intensity of storms could also increase so that there are more major hurricanes (categories 4 and 5 on the Saffir-Simpson scale) with winds reaching speeds greater than 209 km/h.

Since these storms are fuelled by ocean heat, warmer ocean conditions will influence their intensity and longevity. This may enable them to travel farther over ocean water at higher latitudes, and farther across the continent after they make landfall.

With global sea level rise expected to continue to accelerate through the 21st century, the impacts of coastal flooding from tropical cyclones is also expected to worsen.

More in the link below including:

Atlantic hurricanes

Damage to coasts

Future Impacts

https://theconversation.com/hurricanes-to-deliver-a-bigger-punch-to-coasts-113246

Also at: https://www.nakedcapitalism.com/2019/03/climate-change-hurricanes-to-deliver-a-bigger-punch-to-coasts.html

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