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Originally Posted by Solanum
I studied geology at university (though in a limited way and it was a fair time ago), I studied chemistry at university (again limited) and I am a plant biologist by profession. One thing I can tell you is that the vegetation has most certainly changed significantly in the time you claim it hasn't. One of the largest impacts has been the evolution of grasses, which is much more recent and has totally changed the vegetation in large parts of the world.
Of course the paleoclimate informs us in how the climate works, who would deny that. But it is disingenuous to talk about 500 million years ago and how we are colder now. The point is that for the last several hundred thousand years the climate has been relatively stable (glacial/interglacial cycles), and all the measurements we have indicate that we are now going off that stable range. Why?
The best guess we have is that it is due to anthropogenic CO2 emissions. So do you consider that we are currently warming or not? If you consider that we are, then what do you think the cause is?
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I didn't say that the vegetation hasn't changed. I said that even though the "players" have changed, the "stage" and the processes involved are much the same as they are now. There are differences (most notably atmospheric oxygen content), but the physics of the processes driving the climate is the same.
The climate for the past 2.5Ma hasn't been stable at all and claiming it has been is not correct. If it was stable, there'd either be continuous ice or none at all. The climate has fluctuated wildly over the last 2.5Ma, with much of the time being colder than at present (or what is "normal" for interglacials). Even during the interglacials, the climates have gone up and down like a yoyo...the present interglacial being a very good example (even more so than the last two examples).
I never denied that the earth is warming...all I am saying is that there are other factors, just as important as a rise in CO2 which have contributed to the warming and most are man made. Instead of blaming it all on the CO2 why don't they look at some of these other factors...like the increasing spread of plant monocultures in agriculture over the last 100-200 years, the great increase in the size and spread of urban areas and the materials they're constructed out of, the cutting down of the forests, the huge increase in the numbers of cows on the planet (there's 1.5 billion of them...1/5th of them in India alone...remember methane) etc. You're a plant biologist...what happens when you replace a complex ecosystem (like the grasslands) with a monoculture of, say, wheat, over vast areas of your landmass. What you get, apart from all the usual problems in diversity, nutrient fluctuations etc etc, is a raising of the average temperature in the atmosphere surrounding the wheat. The IR flux off the wheat is higher than off the original ecosystem. Take temp measurements before and after the wheat is planted and you'll see the difference. You also get bare soils at some stage...bare soils get hot and re-radiate the heat, the also have higher CO2 output than a normal grassland or forest because of the exposed bacteria load in the soil. So there's a double whammy. Cut down the forest (forest cover on the planet has dramatically decreased over the last 100-150 years) and the same thing happens...temps rise. Not only that but you also get changes in climate patterns which spread worldwide. It's happening with the Amazon and the SE Asian forests and climate right now.
Look at where most of the temp measurements have been taken in the last 150 or so years...in and around towns and cities. As you'd be aware of, towns and cities are usual quite a bit hotter than their surroundings and have their own peculiar microclimates, which also effect the surroundings. Increase the size and extent of those towns and cities over time. Change the materials out of which they're constructed, and what happens?? The urban heat islands grow larger and hotter and affect their surroundings even more so. What does that do to the measurements you take??
You cannot divorce local or regional changes in climate from the overall global system. They're intimately interconnected. For example, the temperatures in much of China have gone up ever since they deforested and de-grassed much of the country, especially the northern half. Now they're having greater problems with desertification and rising temps than they've ever had beforehand. The same is happening in the Amazon. if they keep cutting it down at the rate they are, in 50 or so years you're going to have desertification and rising temps there. Not only that, it's one of the two major heat engines of the planet...and it's lungs as well. What happens when it disappears is going to make the rising CO2 levels look quaint.
Hopefully you can see what I'm trying to get at...the problem is far more than just CO2. By rights, if it was just CO2 rise, then during the Ordivician Period, there shouldn't have been a major ice age. The CO2 levels were around 4200ppm. even though Gondwana was over the Sth Pole, it shouldn't have had ice...but it did, lots of it. The average global temps at the time were only 2 degrees higher than now. So what was keeping the planet that cool, given the high CO2??. It wasn't a different orbit or whatever...they were pretty much the same as now. The Sun was only 3-4% dimmer than now...we get greater differences now, between summer and winter solar insolation than that. You have to look at the rates of erosion and the chemical/hydrological processes involved, the physical positioning of the continents and how they affect global sea and air circulation patterns and host of other things. You also have to remember there were bugger all land plants at the time...maybe some "pond scum" if you were lucky. That would greatly affect the amount of CO2 present in the atmosphere.
See what I mean...CO2 can be important over large times scales, but there's more to short and medium term climate change than just an increase in CO2. Other factors come into play...and at present much of the other things we're doing is only making things worse.