Plants, both through decay and respiration, are responsible for over half of the world’s annual carbon dioxide emissions.
I mention this fact for one reason and that is to communicate the power of the biosphere upon the atmosphere.
Usually, when discussed in the context of climate change, we talk about the opposite phenomenon, which is the power of the atmosphere on the biosphere. Specifically, the increased growth rate of plants in response to increased levels of atmospheric carbon dioxide.
But that focus on the biological benefits of carbon dioxide has limited our view. How does the biosphere affect the climate? Or, to be more precise, how does the changing biosphere, in response to carbon dioxide emissions, affect the climate?
I would like to propose a new climate feedback for the IPCC. Unfortunately for their organization this is a negative climate feedback. My proposition is a simple one based on these facts:
1) As carbon dioxide levels increase plants need to keep less stomata (pores) open to absorb adequate levels of carbon dioxide
2) With less stomata open plants will lose less water due to transpiration
3) This means less water vapor in the atmosphere – less of a powerful greenhouse gas (WUWT?).
Simply put, higher levels of carbon dioxide, all other things being equal (which we know they aren’t), will decrease levels of water vapor in the atmosphere and therefore the greenhouse effect. This effect will obviously be shown above land rather than the oceans, but then again, the atmosphere above the oceans is already very humid, far overpowering carbon dioxide.
Is this a powerful effect? I don’t know. But keep in mind the first sentence in this article before dismissing its magnitude out of hand. It is certainly something I’d like to see tested under strictly controlled conditions.
The effect would probably be logarithmic in nature, just like the radiative forcing of carbon dioxide is logarithmic, because plants can only grow and store so much water in the limited amount of real estate we have. However, plants do have another interesting, but somewhat predictable response to carbon dioxide – they increase the size of their root systems.
Since their leaves are more capable of supplying the needed carbon the limiting factor for growth becomes the minerals extracted by their roots.
Solution: Grow more and deeper roots.
That is one way the extra water might be stored.
Of course, in his book, A Many-Colored Glass, Freeman Dyson discusses the effects of these more intricate root systems. He postulates that as plants grow more roots and less shoots that their ultimate decay will return more of the carbon to the soil and less to the atmosphere.
That could certainly throw a damper on their multi-century long predictions of atmospheric carbon dioxide levels, especially, and again I defer to Dyson,when he calculates that half of the landmass on Earth would only have increase in thickness by 1/100th of an inch per year in order to absorb every last drop of our carbon dioxide emissions.
This example was meant to demonstrate how little we know about the climate and, more importantly, the things we didn’t even realize we didn’t know. How many more mysteries are out there befuddling climate models and their predictions?
As simple as this example is, it is still far more complex than the calculations regarding carbon dioxide.
Or is it?
When clouds precipitate away their moisture, releasing heat towards both Heaven and Earth, wouldn’t the carbon dioxide reflect some of that heat back up towards space? Or would those conditions necessarily require a high amount of water vapor in the air which would drown out that effect?
I don’t know. Maybe it is a stupid question – or perhaps just another unknown unknown.