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Tropics Expand

Re: Tropics Expand

Unread postby dohboi » Sun 13 Nov 2016, 13:39:02

Thanks for this dis and T. (T, you must have missed my intended understatement--when I said "I doubt...smoothly linear" I meant, "There's essentially no conceivable way it could be smooth or linear" especially as it is experienced locally.

But maybe dis could chime in on the physics of these patterns. Any idea of how many possible formations are possible? Will the Hadley cell just keep expanding steadily? Is there a point when it necessarily will break into some other pattern. How long can the three-cell-per-hemisphere hold up? What is likely to go first? Are two-cell structures even physically possible?

(Sorry to pepper with questions. Take any one that seems remotely interesting or move on to other things, as you wish. Thank ahead of time for any light anyone can throw on this. My understanding is that modeling wave patterns even in fairly straightforward contained simple structures can be devilishly difficult. Butterflies and all that...)
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Re: Tropics Expand

Unread postby dissident » Sun 13 Nov 2016, 17:25:34

dohboi wrote:Thanks for this dis and T. (T, you must have missed my intended understatement--when I said "I doubt...smoothly linear" I meant, "There's essentially no conceivable way it could be smooth or linear" especially as it is experienced locally.

But maybe dis could chime in on the physics of these patterns. Any idea of how many possible formations are possible? Will the Hadley cell just keep expanding steadily? Is there a point when it necessarily will break into some other pattern. How long can the three-cell-per-hemisphere hold up? What is likely to go first? Are two-cell structures even physically possible?

(Sorry to pepper with questions. Take any one that seems remotely interesting or move on to other things, as you wish. Thank ahead of time for any light anyone can throw on this. My understanding is that modeling wave patterns even in fairly straightforward contained simple structures can be devilishly difficult. Butterflies and all that...)


The complex question is what sort of new regimes of coupling between the Hadley circulation and the subtropical baroclinic eddy shedding regime. For example, the 2010 super-block that led to fires across Russia and flooding in Pakistan is a new type of mode that is likely to occur more frequently. There is a limit to the expansion of the Hadley cells. In the short term limit the process is quasi-linear but there are fundamental dynamical limits on how far it can expand. This is imposed by rotation of the planet and baroclinic instability. This baroclinic breakdown zone will not just move poleward. It will stay in place and get more intense.

So I do not expect that the tropics will keep on expanding and bringing tropical weather to higher latitudes. Instead we are headed for more extreme mid-latitude circulation regimes which are highly non-linear. Cid is right that the storm tracks will shift poleward. But it is a mixed bag. At the same time there will be highly disrupted weather in the 30-40 degree latitude belts. Of course that does not mean that agriculture will survive. The dry belts on the flanks of the Hadley circulation will not necessarily get drier and larger. But the extreme weather states will amplify.
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Re: Tropics Expand

Unread postby dohboi » Sun 13 Nov 2016, 23:32:31

Thanks, dis.

"there are fundamental dynamical limits on how far it can expand. This is imposed by rotation of the planet and baroclinic instability."

That's what I had heard elsewhere. I don't pretend to completely grok the physics behind this, but about how far can the Hadley expand till it bumps up against these physical limits. Do we know with any precision? And can we estimate when this halt in expansion will happen based on how fast it is expanding now?

It seems like such questions would be of vital interest to those trying to make preparations for these now pretty much inevitable events...oh, I forgot...pretty much no one in charge shows much sign of giving a sh!t about any of these developments...oh well. :/
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Re: Tropics Expand

Unread postby jedrider » Mon 14 Nov 2016, 01:35:47

Coffee And Climate Change: In Brazil, A Disaster Is Brewing


http://www.npr.org/sections/thesalt/2016/10/12/497578413/coffee-and-climate-change-in-brazil-a-disaster-is-brewing

I like coffee, so what a bummer.

Tropics expand and subtropics dry out, is that how it works (don't know, just jumping into this discussion without much research)?
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Re: Tropics Expand

Unread postby dohboi » Mon 14 Nov 2016, 23:52:14

And it's not just coffee.

Cocoa, tea, wines...lots of our favorite beverages and sweets are threatened by rapidly shifting climate zones.

Not sure what kind of threats hops might be under, but it could really start a revolution if CC started threatening basic ingredients of beer! (Esp. among we hipsters who are gaga over super hoppy IPAs ! :) :) :) )
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Re: Tropics Expand

Unread postby dissident » Tue 15 Nov 2016, 00:34:41

dohboi wrote:Thanks, dis.

"there are fundamental dynamical limits on how far it can expand. This is imposed by rotation of the planet and baroclinic instability."

That's what I had heard elsewhere. I don't pretend to completely grok the physics behind this, but about how far can the Hadley expand till it bumps up against these physical limits. Do we know with any precision? And can we estimate when this halt in expansion will happen based on how fast it is expanding now?

It seems like such questions would be of vital interest to those trying to make preparations for these now pretty much inevitable events...oh, I forgot...pretty much no one in charge shows much sign of giving a sh!t about any of these developments...oh well. :/


I can't give you a latitude number since there is no published estimate of it. The key is that the air parcels moving off the equator (or near it) on the upper parts of the two Hadley cells conserve absolute angular momentum to high degree. This results in the formation of strong westerly jets as these parcels head poleward and downward. If one preforms a thought experiment where the two Hadley cells expand into middle latitudes and the baroclinic eddy formation zones move away from their current location, then the intensity of the subtropical zonal jets would be absurdly high. Such monster winds by the nature of how they are maintained would have a very large vertical gradient (shear) and would be thus super-unstable to baroclinic instability. The conclusion is that there will be baroclinic adjustment in the same region with perhaps some broadening in latitude no matter how much warming there is.

One can conceive of a way to get around the above constraint if some process were to make the Hadley cells progressively weaker as they expanded and including zonal momentum damping on the upper parts of the circulation cells which would reduce the spin-up of the subtropical jets. But there is no indication of such a regime emerging.
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Re: Tropics Expand

Unread postby dohboi » Tue 15 Nov 2016, 07:12:40

OK, thanks.
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Re: Tropics Expand

Unread postby Tanada » Thu 27 Sep 2018, 22:34:09

More plus pictures at link below quote.

The tropics are widening rapidly, but humans may not be entirely to blame — yet

An uptick in tropical expansion the past few decades would seem to suggest that some unknown factor, perhaps as a result of human activities, is driving the widening of the tropics. But a study led by Paul Staten, an atmospheric sciences professor and researcher at Indiana University Bloomington in the United States, finds that that is not necessarily the case.
Staten and his colleagues determined that the tropics have been widening at an average rate of about 17 miles, or 0.2 degrees latitude, per decade in both the Northern and Southern hemispheres, which is not outside of what climate models predict.
Staten and team state in the study that no hidden forcing is required in order to explain the tropical expansion we’ve already observed — our current models, which take into account natural variation and man made global warming, can account for the 0.2 degree-per-decade expansion rate they established.


Scientists have observed the tropics expanding toward Earth’s poles in recent decades, which was projected to happen as increasing concentrations of greenhouse gases in the atmosphere continue to heat up the planet. Some observational studies have found that the tropics are widening much faster than climate models predicted, however.

Any widening of the tropics can have significant impacts for the roughly half of the global population who live there — from shifting rain belts and growing desertification to more severe and frequent droughts and wildfires. So it’s an important question: Is anthropogenic climate change causing even more rapid poleward expansion of the tropics than expected? And if so, why?

An uptick in tropical expansion the past few decades would seem to suggest that some unknown factor, perhaps as a result of human activities, is driving the widening of the tropics. But a study led by Paul Staten, an atmospheric sciences professor and researcher at Indiana University Bloomington in the United States, finds that that is not necessarily the case.

“[S]ome studies claim that the observed tropical widening outpaces that expected from modern climate change, suggesting that some ‘hidden forcing’ may be unaccounted for. Here we strive to resolve this apparent mystery by synthesizing results from the growing body of literature on the quantification, attribution and underlying processes of tropical widening,” Staten and team write in a paper detailing their findings, published in the journal Nature Climate Change late last month. “We review metrics, causes, observations and simulations of tropical widening, and find that the widening of the global mean tropical belt may not be predominantly human-induced.”

In contrast to previous estimates of tropical widening made since the beginning of the satellite era in the late 1970s, which ranged from 0.25 to 3 degrees of latitude per decade, Staten and his colleagues determined that the tropics have been widening at an average rate of about 17 miles, or 0.2 degrees latitude, per decade in both the Northern and Southern hemispheres — though they add that the rate can vary significantly from year to year and location to location.

“If we compare the observed trends of how the tropics have widened to modeling trends, it’s actually not outside of what the models predict,” Staten said in a statement.

Staten and team state in the study that no hidden forcing is required in order to explain the tropical expansion we’ve already observed — our current models, which take into account natural variation and manmade global warming, can account for the 0.2 degree-per-decade expansion rate they established. “Including recent evidence, it is fair to assert that the natural swings in decadal atmospheric and oceanic variability may have driven at least as much of the observed expansion as human activity,” the researchers write.

Staten said that this should give us more confidence in predictions based on current climate models. “Climate change should continue to expand the tropics over the next several decades,” he said, adding: “But the expansion may not continue at the rapid rate we’ve seen; at times it may even temporarily contract.”

The researchers focused on five factors that influence the widening of the tropics, including increasing greenhouse gas emissions; ozone depletion in the stratosphere over the South Pole; aerosols from volcanic eruptions; pollution like soot and ozone in the troposphere; and natural variation, such as changes in sea surface temperatures due to El Niño and La Niña events.

Due to how complex these factors are, the researchers note, it’s actually quite difficult to discern between natural and manmade causes of tropical widening. But if we don’t do something soon to rein in greenhouse gas (GHG) emissions and pollution, the human factor will become much more readily apparent.

“Although discerning a forced signal in the observational record is challenging, the detection of such a signal in the future is not so much a question of if, but when, if humanity continues on the business-as-usual path of GHG emissions,” the researchers write in the study. “In the near term, natural variability muddles the widening signal of increasing GHG concentrations. But models project that the forced tropical widening will break out of the envelope of natural year-to-year variability some time in the middle of this century.”


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Re: Tropics Expand

Unread postby Tanada » Thu 27 Sep 2018, 22:36:01

Tropics Expanding with Climate Change



The tropics are expanding outward from the Equator due to the various factors causing climate change, according to a new study.

The thermal patterns since the late 1970s have gradually pushed the arid and semi-arid regions of the tropics by about 17 miles per decade, according to the paper in the journal Nature Climate Change.

“Our synthesis shows strong evidence that the tropics have widened by about 0.5 degrees per decade since the beginning of the satellite era (1979),” write the authors. “Since no one has a crystal ball to tell when the (Pacific decadal oscillation) will switch phases (as decadal prediction remains a major challenge to the climate community), it is impossible to predict whether or not the Earth’s tropical belt is going to continue bulging in the coming decade.”

The estimations have nothing to do with the Tropic of Cancer and Tropic of Capricorn, at 23 degree 27 minutes north and south, respectively. Instead, the estimations were based on observations of the climate patterns established by the Hadley cells, cycles that circulate heat and thermal energy and weather patterns from the Equator outward to the subtropics, and back again, like a natural pump.

The dynamics have been reaching ever outward toward the temperate zones since the advent of satellite observations, the researchers conclude.

The team identifies various factors including greenhouse gas emission, ozone depletion at the South Pole, volcanic aerosols, pollution and natural variation, which may play a part in the changes. The balance between naturally-driven and human-caused changes have yet to be better understood.

One upside: according to their models, the recent “bulge” in the tropics will slow down somewhat, partly perhaps because the ozone depletion over Antarctica has been largely reversed.

But even with fluctuations, the larger trend is clear, the researchers add.

“Climate change should continue to expand the tropics over the next several decades,” said Paul Staten, one of the authors, from Indiana University-Bloomington. “But the expansion may not continue at the rapid rate we’ve seen; at times it may even temporarily contract.”

“Considering that about half of the world’s population either lives in or near these subtropical semi-arid climate zones, the implications of tropical widening are potentially significant,” the paper adds.


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Re: Tropics Expand

Unread postby Cid_Yama » Fri 28 Sep 2018, 18:15:10

Appears to be a follow-up on the OP from 2010.

Cid_Yama wrote:
“THE PROMISE of food lies in the tropics,” the UN Food and Agriculture Organization director general said at the University of the Philippines. “Here in this sun-drenched belt of land, temperature is benign and rainfall abundant. These could be the food granaries for the world of our children.”

Not anymore. Rising temperatures have widened the “Tropical Belt,” notes Nature Geoscience. Since the FAO official delivered his address in Los Baños in May 1979, the tropics expanded by between 2 and 4.8 degrees latitude. As the world warms, edges of the “Belt”—outer boundaries of the subtropical dry zones—drift toward the poles.

Temperature and rainfall changes are altering yields. Affected are politically volatile crops like corn and rice. “In the Philippines, rice yields drop by 10 percent for every one degree centigrade increase in night-time temperature,” BBC’s environment correspondent Richard Black writes.

The slump is region-wide. As droughts dry reservoirs, yields have fallen by 10 to 20 percent over the last 25 years. More declines are ahead.

“We found that as the daily minimum temperature increases, or as nights get hotter, rice yields drop,” researcher Jarrod Welch said. “Where temperature increases more than 3 °C, impacts are stressful to all crops and in all regions,” the Intergovernmental Panel on Climate Change (IPCC) concluded.

There are biological limits to what can be done. “We can’t just move all our crops north or south because a lot of crops are photosensitive,” notes Dr. Geoff Hawtin at the International Center for Tropical Agriculture. “Flowering is triggered by day length.”

We don’t know where the tipping points are,” Hawtin adds. “They could come quite quickly.”
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Re: Tropics Expand

Unread postby Tanada » Thu 17 Dec 2020, 08:35:46

More plus pictures and graphs at link below quote.

Atlas reveals birds pushed further north

Data from 120,000 birdwatchers in 48 countries shows forest birds have expanded their range while area occupied by farmland birds has shrunk

Europe’s breeding bird populations have shifted on average one kilometre north every year for the past three decades, likely driven by the climate crisis, according to one of the world’s largest citizen science projects on biodiversity.

The European Breeding Bird Atlas (Ebba2) provides the most detailed picture yet of the distribution of the continent’s birds after 120,000 volunteers and fieldworkers surveyed 11m square kilometres, from the Azores in the west to the Russian Urals in the east.

The book documents changes in the range of Europe’s 539 native bird species in the 30 years since the first Ebba, which was published in 1997 but was based on observations from the 1980s. It shows that since the first study, each population can be found around 28km further north.

Mediterranean species such as the European bee-eater and little egret are now reaching the UK, France and the Netherlands, mainly due to milder winters. Eurasian bittern, pied avocet and red kites have also expanded their range, probably in response to better protection of habitats coupled with laws banning persecution.

Overall, 35% of birds increased their breeding range, 25% contracted their breeding range and the rest did not show any change, or the trend is unknown. Forest birds and those protected by international legislation have generally expanded their range, while farmland birds occupy a smaller total area.
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Generally, if a species is present in more areas it is less likely to go extinct, but it could be spreading out because of habitat deterioration, and not because the population has increased. “The results are confirmation that the major driving forces are climate change and land-use change. At the same time, the situation is really very complex, and that’s why we will provide this dataset for further exploration and investigation,” said Dr Petr Voříšek from the Czech Society for Ornithology.

r Iván Ramírez, senior head of conservation at BirdLife Europe and central Asia, said: “Those birds that have been legally protected have been doing better than those which are not protected. This is a really important message within the European Union. We have one of the oldest policies – the Birds Directive – and we can prove that it works.” Birds protected by the Bern Convention, such as white-tailed eagles, are also doing better.

As the climate warms, forests are stretching into boreal and Arctic regions. In parts of northern Europe there has also been tree planting (mainly for wood and paper) and land abandonment (specifically in Mediterranean areas) which have damaged farmland birds but benefited many woodland species such as woodpeckers and warblers.

Alpine species are also losing out as scrubby trees and vegetation colonise higher mountain slopes, shrinking the range of mountain grassland specialists such as wallcreepers and water pipits.

Generally, farmland birds are big losers, suffering overall declines in population and reduced distribution because agricultural intensification means there is less food, such as insects and residue from harvesting. The State of Nature in the EU 2013-2018 assessment showed 80% of key habitats were in poor or bad condition, and intensive farming is a major driver of decline. The UK’s farmland birds have declined by 55% since 1970.


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Re: Tropics Expand

Unread postby JuanP » Thu 17 Dec 2020, 08:53:40

I have attended a couple of NOAA and USDA presentations on the shifting of plant hardiness zones boundaries over the last few years. All of South Florida is expected to become an area with tropical climate over the coming decades.
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Re: Tropics Expand

Unread postby REAL Green » Thu 17 Dec 2020, 09:43:33

“The tropics are expanding, and climate change is the primary culprit”
https://www.sciencedaily.com/releases/2 ... ew%20study.

“Earth's tropics are expanding poleward and that expansion is driven by human-caused changes to the ocean, according to new research. The tropics wrap around Earth's middle like a warm, wet belt. This part of the globe gets the most direct sunlight throughout the year and is characterized by high average temperatures and heavy rainfall. In contrast to the tropics' lush interior, however, this region's edges are hot and parched. Scientists have noticed for the past 15 years that these arid bands are expanding toward the poles into regions like the Mediterranean, southern Australia and southern California. Interestingly, these dry areas have expanded more in the Southern Hemisphere than the Northern Hemisphere and researchers have struggled to pinpoint exactly what is driving the trend. A new study in AGU's ,Journal of Geophysical Research: Atmospheres argues that the failure to agree on an exact mechanism has been, in part, because most researchers have been looking in the wrong place. The new study found tropical expansion is driven primarily by ocean warming caused by climate change rather than direct changes to the atmosphere. A bigger shift is happening in the Southern Hemisphere because it has more ocean surface area, according to the new study.”
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Re: Tropics Expand

Unread postby dissident » Thu 17 Dec 2020, 12:47:21

The problem with over-simplification. The tropics are characterized by a semi-annual cycle and not an annual cycle. The stability of the weather is extreme so that there is little need for weather forecasting. The extratropics are subject to an annual cycle and variable weather. That will not change, period. So the "expansion of the tropics" is really and expansion of higher temperature regions. But it sounds calling this tropical expansion is something that journalists would do since they don't have the education. And no, this is not hair splitting. Super-blocks and violent fronts are not trivial differences.

The potential for tornado alley to get worse is there. Stronger zonal winds aloft are a factor for driving tornado formation in regions with substantial deep convection since there is a chimney-like effect resulting in stronger updrafts. If the tropics were really expanding, tornado formation would be suppressed. Thing are more than likely to get nasty in the extratropics due to changes in the weather intensity. It is not just temperature changes.
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Re: Tropics Expand

Unread postby Subjectivist » Fri 18 Dec 2020, 14:26:02

Hope this works, still having login issues.

Here is the thing about the moving of the tropic zone not mention in RealGreens article. While the dry belts are moving away from the equator this is a side effect of the rainy tropical zone expanding. The way I was taught is the Tropics are where the great moist air rises up to the top of the Troposphere layer, then it tips north or south and drops rain as it travels hundreds or thousands of kilometers before it is completely dried out. Then the column belt zone whatever you call it sinks back to the surface as very dry air. The dry air covers a wide swath, at sea it picks up moisture easily but where it is over land, like Arizona, it sucks up all the available moisture from rivers and lakes without getting saturated. That is why Phoenix has like 15% humidity most of the time. Eventually as it travels further from the equator it picks up enough moisture that it rolls over into the green belt/temperate zones. The temperate zones are another upwelling wet zone, then as the air dires it falls back to the surface in the sub polar/polar zone as dry very cold air. That is why it doesn't rain or snow very much at the poles, Antarctica is technically a cold desert ecosystem.
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Re: Tropics Expand

Unread postby dissident » Fri 18 Dec 2020, 16:59:18

The descending flanks of the Hadley circulation maintain a high pressure zone that contributes to the lack of rain. The so-called ITCZ where the ascending part is (it bounces around the equator in a 20 degree band) does remove a lot of water through precipitation. But the Hadley cell cannot expand arbitrarily. It is limited by the fact that the Earth is rotating so it is broken up on its poleward flanks by baroclinic instability. Weakly rotating Venus has something like a global scale Hadley circulation but since it is slowly rotating there is a maximum heating zone on one side of the planet. So it is not zonally symmetric.

The extratropical baroclinic zone will be there as long as the planet rotates and has an atmosphere. The extra thermal energy in the system will simply drive more baroclinic adjustment. There was and probably still is a false notion in the atmospheric science community that the pole to equator temperature gradient drives the adjustment so polar warming will somehow suppress it. This is a simplistic view based on toy model based stability analysis. In the real world the origin of baroclinic instability is the Hadley circulation which drives the subtropical zonal jets. These jets do not depend on any pole to equator temperature gradient but are generated through the transport of near equatorial angular momentum to the subtropics which spins up westerlies. This process will not be deleted by any warming. It is a fundamental dynamical feature. The ITCZ deep convection will make sure that no acceleration of tropical easterlies will occur since this convection helps to transfer the surface momentum through the tropical atmospheric column.

The subtropical jets are perpetually baroclinically unstable since they are perpetually being generated by the Hadley circulation. This primary baroclinic instability results in eddies that pump both heat and kinetic energy to the extratropics. The net result of these eddies is to spin up a secondary westerly flow in the middle latitudes. This westerly flow is also subject to baroclinic instability. So polar warming is not able to make this source of heat and momentum go away. Instead what it gives us is the breakdown on the polar front jet in the polar fall and early winter that enables more heat transport by barocolinc eddies towards the poles. Before the warming and loss of sea ice the Arctic polar cap was shielded by a dynamical wall of westerlies around the polar night terminator. In the case of Antarctica, it has not experienced the sort of warming thanks to its high albedo and lack of underlying waters that could remove it. There is no melting sea ice sheet there. The circumpolar ocean current around Antarctica also plays a big role in maintaining the cold in the southern polar cap.

Global warming will not shut down the Hadley circulation and the baroclinic instability that it drives. The southern hemisphere roaring 40s are going to be there until the continents redistribute themselves.
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Re: Tropics Expand

Unread postby JuanP » Fri 18 Dec 2020, 18:00:01

Thanks dissident! I can tell from the clarity of your explanation that you know what you are talking about. You have to really understand a subject to be able to explain with such ease, making complex processes that are alien to most of us seem intuitively simple to understand.
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Re: Tropics Expand

Unread postby REAL Green » Sat 19 Dec 2020, 07:05:21

It is the parched borders of the tropics growing not the lush parts. This spells trouble for many in these narrow expanding belts. This is especially true for human habitation becuase of wet bulb effect.

“The mystery of the expanding tropics”
https://www.nature.com/news/the-mystery ... cs-1.19271

“One spring day in 2004, Qiang Fu was poring over atmospheric data collected from satellites when he noticed an unusual and seemingly inexplicable pattern. In two belts on either side of the equator, the lower atmosphere was warming more than anywhere else on Earth. Fu, an atmospheric scientist at the University of Washington in Seattle, was puzzled. It wasn't until a year later that he realized what he had discovered: evidence of a rapid expansion of the tropics, the region that encircles Earth's waist like a green belt. The heart of the tropics is lush, but the northern and southern edges are dry. And these parched borders are growing — expanding into the subtropics and pushing them towards the poles. Cities that currently sit just outside the tropics could soon be smack in the middle of the dry tropical edge. That's bad news for places like San Diego, California. “A shift of just one degree of latitude in southern California — that's enough to have a huge impact on those communities in terms of how much rain they will get,” explains climate modeller Thomas Reichler of the University of Utah in Salt Lake City.”
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