#Climate Why forests are important @LaLaRueFrench75

This is a quick explanation to follow up a tweet to me from a lady Laura LaRue who goes by the handle of @LaLaRueFrench75
We’ve all seen the image of the “sweating earth” used to portray the effect of “greenhouse warming”, but whilst the actual effect of CO2 is so small it’s probably akin to putting a hanky on your head, the image does show an important part of the earth’s heat control system.
The important thing to understand is that because the earth is in the vacuum of space, the earth can’t lose heat by “sweating”. Instead it must all leave through infra-red radiation. Something we can all experience by going out on a cold cloudless night when the sky feels to be “sucking out” the heat (although what we actually experience is the lack of back-radiation from the clouds.
Around half the heat leaving the surface and going up into the atmosphere (high enough so that it can then lose it’s heat above the clouds) does so from plant evaporation. So, roughly half the heat transport is regulated by plants. As a result, even a small change in local plant cover, and hence local plant evaporation, can change temperature. And if we go from forest with near saturation plant cover to a city with almost none, then the effects are quite dramatic with cities being several degrees warmer.
The problems with water
However, the water still comes down in rain. And because there are no longer the plants to suck up the water, and because many urban areas are concreted and tarmacked, the result is that, unlike the slow trickle from a Forest, in cities, there is a dramatic rush of water down into the rivers every time it rains. And those rivers in turn tend to be straightened, and dredged and measures made to stop flooding forcing more and more water to rush downstream in one almighty surge (which sooner or later means massive flooding lower down the rivers)
But, it gets worse! Because as we move to farmland and urbanised areas, we drain all the bogs. That doesn’t mean less water flows from them. Instead, it means that water that used to be stored in bogs, marshes, ponds, etc. now flushes straight into the rivers. As a result of this, the local water table drops, this in turn means the area is more arid, which often means farmers have to take water from rivers etc. which in turn can drop the water table further.
So, locally, changing farm use, both leads to massive floods down stream, but also arid conditions later. (In the past, we’d have small floods at the top of rivers, now it all gets channelled down to the flood plains – which are often the locations of big population centres!)
The Effect on Global temperature
However, whilst all these effects are local, it is also true that globally, man-kind has been turning many natural areas into farmland and urban areas. Firstly, because most weather stations are located in convenient areas near population centres, and because these tend to have seen the most change in land-use, there is no doubt whatsoever, that this effect has significantly added to the “average” global temperature. Because the value we get, is not so much “average” of the globe, but instead “average” of those areas close to old urban centres, which were most affected by changes in land-use last century.
However, just because global temperature over-represents those areas where changing land use has increased temperature, it does not mean the average global temperature was not also caused to rise from this effect. So, it is more than likely global temperature did rise directly from changes to land use (mainly because this has changed so extensively across the world). However, such changes are a step. Once land use changes, it does not cause further warming.