What a shame it is that the entire foundation of AGW is built on human carbon dioxide emissions. After all, global temperatures peaked in 1998, and have remained steady or declined since then...and all the while carbon dioxide has been increasing. If human carbon dioxide causes global warming, then it can't just "take a breather"; that's thermodynamically impossible.
But proponents of AGW "theory" don't give a rat's ass about the laws of thermodynamics.
If global warming were due to human carbon emissions, we would not be seeing oceans which are cooler now than in 1998; nor would we have seen air temperatures remain steady or decline, because we're putting out more carbon than ever. If more carbon means more warming, there is something wrong with the theory, because there's more carbon and there hasn't been more warming for ten years.
In fact, there's plenty of evidence to suggest that man-made carbon has little effect on the global environment; doing anything to cool Earth may trigger an ice age, which would be an ecological disaster of extreme magnitude.
So let's look at the ideas presented in the article, shall we?
a) The caption for the picture with the article says, "The tubes depicted running down skyscrapers are photo bioreactors, which, as envisioned, would take in CO2 and produce an energy-yielding biofuel. (Institution of Mechanical Engineers)".
..."[P]roduce an energy-yielding biofuel". We've already seen that biofuels aren't energy sources--it takes about a gallon of petro diesel to make a gallon of bio diesel--and so these "bioreactors" would probably be net consumers of energy, one way or another. To make matters worse, then, the carbon dioxide scrubbed from the atmosphere by these bioreactors would be re-emitted when the fuel produced was used. In other words, you're putting the carbon right back into the atmosphere. And there's no good evidence to suggest that this would have any measurable effect on the global climate.
This idea ends up spending money to do, essentially, nothing about the global climate.
b) "[B]illions of tiny mirrors, launched into orbit to deflect solar rays away from Earth". Okay: reducing solar insolation at the Earth's surface will definitely cause cooling.
But what do you do when the temperature has fallen far enough? You can't just switch off "billions of tiny mirrors". The idea is apparently driven by the notion that solar insolation will have to be reduced ad infinitum in order to maintain "historic" temperatures. This is probably wrong; and if you want to turn this thing off, you can't. So if the Earth gets too cool, we're out of luck.
c) "[B]ig, fluffy clouds, artificially whitened so they reflect more sunlight back into space". How do they plan to artificially whiten clouds? Bleach? Says here "sulfate aerosols", which would somehow damage the ozone layer. Any figures on how much "sulfate aerosol" it would take to whiten enough clouds to reflect enough sunlight back to space to cool the Earth measurably?
And how do we get those "sulfate aerosols" above the clouds in the first place without burning all kinds of fossil fuels, completely negating the effect of reducing solar insolation in the first place? (If you agree that human carbon emissions cause global warming, that is.)
I want a number: how many megatons of "sulfate aerosols" will it take to reduce the Earth's temperature by 0.1°C?
d) "[M]echanical trees, ugly but effective at sucking carbon dioxide from the air along busy highways". Okay, here's a crazy idea: what about REAL trees?
"100,000 'trees,' at a cost of $20,000 each, could in theory scrub Britain clean of a significant portion of its carbon emissions." Do you know how many real trees you can plant for $20,000?
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These ideas are all "back-of-the-envelope" ideas, of course. These people are 'way ahead of the curve, though; their automatic buy-in to "global warming is caused by human carbon emissions" is problematic because the global climate is an extraordinarily complex thing, and we don't understand how it works.
AGW can be demolished by the application of three events, two historic and one current: the Medieval Warm period, the Maunder minimum, and the Cycle 24 Minimum we're seeing now.
The global climate was much warmer during the Medieval Warm Period than it is now; and carbon concentrations in Earth's atmosphere were much lower than now--100 ppm lower, or more.
The Maunder Minimum was a period of bitter cold winters, and it occurred as the Industrial Age was taking its first faltering steps. Sunspot activity was virtually nonexistent for decades, indicating a period of decreased solar output.
The Cycle 24 minimum is taking place now, and has been underway for nearly eighteen months. In 2008 NASA's Goddard Institute for Space Science recorded a drop of 0.775° in the global temperature anomaly without a corresponding drop in atmospheric carbon. Furthermore, the oceans are getting cooler; if the oceans were absorbing heat from the air, they would be warming. And vice versa: if the oceans were losing heat to the atmosphere, the atmosphere would be warming. Both are cooling--so where is that heat going?
We don't know what our climate is doing, nor do we know why.