Global warming = desertification... Why?

[Starthane Xyzth]

I'm sure I've missed a good few similar discussions on SciForums in the last 2 years, but I just have to ask.

Here's an article published recently in The Sun newspaper, showing a fairly standard prediction of how the Earth might look after a 4 Centigrade temperature rise.

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Note the prevalence of deserts - not merely spreading towards the poles, but spreading also right across the equator, obliterating rainforests and savannahs. :bawl:

Deserts are created more by the absence of water than by high temperatures. I am being too naive in assuming that, if the World overall is hotter, there will be more evaporation from the oceans - and hence more rain, not less? I would expect a climate conducive to the spread of jungles and swamps, much as the Earth was in Mesozoic times. :shrug: (Even if people have cleared all the remaining tropical forest by 2050, they will still farm the land as long as it rains).

As recently as the Eemian interglacial, the Earth was considerably warmer than today and sea levels were higher. If most of our land surface had been desert back then, we would have far less biodiversity today - especially in the tropics.

Does anyone know a reason why manmade global warming should produce such a dry and hostile world? Or are scenarios like this based on unscientific scaremongering?

 

[iceaura]

Deserts are created more by the absence of water than by high temperatures. I am being too naive in assuming that, if the World overall is hotter, there will be more evaporation from the oceans - and hence more rain, not less?

More evaporation does not equal more rain, at least not in any equivalent amounts in the same area.

Hotter air picks up more water as gas, and holds it as gas - less of it condenses out into rain nearby, and more of it remains in the hotter air, after having been removed from the soil, plant leaves, lakes, etc, of dry places (places with already drier air). It then blows away to places where it can cool and condense out, far away.

In the western US you can sometimes see rain clouds that have cooled and condensed high up raining down into areas of hotter air below them, and watch the rain evaporate right into the air, never reaching the ground.

 

[Buffalo Roam]

A simple question? when did the world have the most jungle/rain forest cover?

 

[matthyaouw]

The sun isn't exactly an authority on climate issues. Or indeed any issues. Personally I would want to find this map in a more reputable source before I pay it too much notice.

 

[lightbringer]

A simple question? when did the world have the most jungle/rain forest cover?

probably some time during the Devonian. All it is when seed bearing plants first came about there wasn't really that much to eat them.
though if you wanted more true foresty type things and not merely plants in large groups I believe that would be the Jurassic

 

[Starthane Xyzth]

The world's forest cover would probably have been greatest in the Palaeocene, a few million years after the last mass extinction - warm wet climate, and no large herbivores to eat the tree saplings.

More evaporation does not equal more rain, at least not in any equivalent amounts in the same area.

So we could have a hot Earth with steaming equatorial sea, but still less precipitation than today?

 

[iceaura]

So we could have a hot Earth with steaming equatorial sea, but still less precipitation than today?

Less in some places, more in others. Some big deserts where there are cornfields now.

Problem is, we're set up according to the current pattern.

 

[Megabrain]

article published recently in The Sun newspaper, showing a fairly standard prediction of how the Earth might look after a 4 Centigrade temperature rise.

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The SUN eh? that's the paper with a naked model on Page three, devotes 40% of it's pages to soap 'stories' 35% to (Ugh) football, and the rest to advertising tv listings etc - If the SUN said the world was round they would be wrong (it's more a slightly flattened sphere anyway).

It's a tabloid, from the same stable that gave you "London bus found on the Moon", "Elvis ate my hamster" and many others....

 

[Starthane Xyzth]

It's a tabloid, from the same stable that gave you "London bus found on the Moon", "Elvis ate my hamster" and many others....

Not quite: you are paraphrasing The Sunday Sport, which doesn't even pretend to represent reality. The Sun are a little more sober than they.

 

[fedr808]

Maybe its because since there is more heat, that means that there is a significant increase in evaporation. So some areas could theoretically become desert because they get so much sun and heat that the water is dried up.

 

[Xylene]

I'm more concerned about the geological chaos that could be unleashed by global warming. Consider this scenario--Greenland loses all its ice-cap; isostatic rebound of the crust causes earthquakes all over the northern hemisphere due to the rearrangement of the asthenospheric material between the crust and upper mantle.

East and West Antarctica lose all their ice; same scenario in the southern hemisphere.

 

[Buffalo Roam]

Now how about the fact that we are past the minimum eccentricity of the orbit of the Milankovitch Cycle, and are now moving to the higher eccentric orbit;

the difference between closest approach to the Sun (perihelion) and furthest distance (aphelion) is only 3.4% (5.1 million km). This difference is equivalent to about a 6.8% change in incoming solar radiation. Perihelion presently occurs around January 3, while aphelion is around July 4. When the orbit is at its most elliptical, the amount of solar radiation at perihelion is about 23% greater than at aphelion. This difference is roughly 4 times the value of the eccentricity.

and as the orbit will now continue to gain more eccentricity to a a difference of 23% in incoming solar radiation at the deep end of the cycle, and we end up in a Ice Age, and nothing we can do is going to stop it, approximately 75,000 years from now, just as we couldn't stop the Milankovitch Cycle from entering a minimum orbit of eccentricity, and the highest point of incoming solar radiation.

Our weather report for tonight is for a hard freeze, 25 F or -3.889 C and it is May 17.