Water, Water, Everywhere - Bumps and Wiggles (6):
Introduction: This is the sixth in a series on understanding climate variability, global warming, and what we might do about it. The series focuses on the past 30 years and the next 30 years. Sorry about the hiatus of the WU “expert” climate blog; it’s been a little too busy. There are 2-3 more in the series, then on to something different.
For those who came in late, and might care, the basic idea of this series of blogs is that over the next ten years or so, one of the primary focuses of climate scientists should be to better quantify the variability of the global-average surface temperature. There are a number of reasons for this focus. 1) It brings attention to the “processes” that are responsible for variability, which will help build the foundation for climate forecasts. 2) It is simply no longer adequate to simply say that – given the observed natural variability - that any discrepancies between existing projections and observations are, formally, small. They are noise.
I keep building up this figure. One of my first points was that it is in our best interest not to forget that We the People are in the middle of this climate system, and that when we talk about climate change and its consequences we are, in the end, talking about what climate change means to us.
Figure 1: Simple Earth 4: People, Sun, Volcanoes, and Ocean Heat.
With that in mind we have spent some time following the heat into the ocean; took a distracting turn because of some spurious claims about the Icelandic volcano; and briefly mentioned that we had to revisit the Sun (Remember those little dots on the Sun suggest variability, and I have drawn that wiggly line to remind us.) Ultimately, next blog, I will get back to an old subject, El Nino, Pacific Decadal Oscillation, etc. This blog will focus on something direct, things that change the radiative forcing in the atmosphere.
The whole concern about global warming is the increase of “radiative forcing” in the atmosphere due to the addition of carbon dioxide from combustion. That is the greenhouse effect, and carbon dioxide holding heat close to the surface of the Earth. When we start to worry about the bumps and wiggles in the observations, then we need to think about ways other than carbon dioxide that change the radiative forcing. There is a set of greenhouse gases, nitrous oxide, methane, and the chlorofluorocarbons which are often mentioned. (There is a good entry on greenhouse gases in Wikipedia
.) These gases are the ones most associated with being caused by humans. I, however, want to focus on water vapor.
Water vapor is at the core of many of the distracting arguments about global warming. Water vapor is the “most important” greenhouse gas, when most important is defined as what contributes most to the heating of the Earth’s surface. Without greenhouse gases, the Earth’s surface would be close to zero degrees F. Water vapor and naturally occurring carbon dioxide are responsible for the observed surface temperature being closer to sixty degrees F. (Again, Spencer Weart’s great book
.) Water is often stated as being responsible for 2/3 rds of the warming.
Water vapor is special, because within the climate it is vigorously cycled and recycled between the ocean and the land and the ice sheets. And it changes phase: gas, water, and solid. The atmosphere is responsible for a lot of the cycling. There is so much water in the ocean, that it always acts as a source of water for the atmosphere. So if it gets warmer, there is more water vapor moving from the ocean to the atmosphere. This, of course, increases the greenhouse effect of water vapor. I have tried to suggest this in the next figure.
Figure 1: Simple Earth 4: Water evaporating from the ocean. Rain back. And some water getting transported up into the stratosphere (the cold spot).
In the figure I have suggested the increased evaporation from the ocean by the big blue arrows. I also suggest the return of water to the surface by the big dotted arrows; that’s rain.
So if we think about it a little, as the Earth warms there will be changes in the water vapor and these changes will impact how much water vapor heats and cools the Earth. Of course, the first way we expect water vapor to behave is to enhance the warming. One paper that I like is by Francis and co-authors
. This paper investigates from a process point of view what is happening in the energy balance of the Arctic sea ice. There is a lot going on with Arctic sea ice, ranging from heat transport by the atmosphere and the ocean, to variability of runoff by the rivers of Canada and Russia. The Francis paper finds that there is a large change in the heating due to the increase of water vapor and clouds. This increase is related to not only it just being warmer, but also to their being less sea ice. Hence, it is easier for water to get from the ocean to the atmosphere. (Here is a far more comprehensive discussion by Santer et al.
of how water vapor has changed due to carbon dioxide increases.)
If we think about water vapor a little more, however, we come to a situation that is not so intuitive. Back in January Jeff Masters
did a thorough report on a cool paper by Susan Solomon et al.
. They showed enhanced cooling due to water vapor changes. Cooling? Well it is cooling that is realized by slowing the warming due to carbon dioxide.
What is happening here? Back to my figure, in the tropics air moves upward due to the Hadley circulation (I can use that without explanation?) The Hadley circulation
is rising air in the tropics due to heating at the surface and, well, “hot air rising.” As air rises it cools. The higher the air rises the cooler it gets. The colder the air gets, the less water vapor it can hold. I have tried to represent this rising air by the two dark blue lines and the region labeled “cold” in the figure. What Solomon et al.
argue is that that cold region has gotten colder. This leads to less water at high altitudes. This reduces
the greenhouse effect of water. Because this change takes place at high altitude, where there is very little water and it is very cold, it has a much bigger impact that a similar change would have at the surface.
So here is a change that for the last decade or so has helped to counter some of the warming associated with increasing carbon dioxide. So some heat goes into the ocean, the stratosphere changes – yes, there are ways that the Earth responds that cools things relative to that fellow standing under the apple tree.
The Solomon paper opens up a lot of interesting questions. Why has it gotten colder up in the part of the atmosphere marked "cold" in Figure 2? One reason it could get colder is because of increased heating at the surface, which causes the air to rise higher. Higher rising air equals colder air. Therefore, this change in stratospheric water is not in any way a challenge to the basic fact that increasing carbon dioxide increases surface temperature. (For those who want more, here is a paper by Santer et al.
that discusses the tropopause rising.)
Another question that Solomon et al. might motivate is – will the Earth heal itself? Can this cooling act to counter the warming? There are, indeed, ways that the Earth will respond that will include “cooling.” All evidence that we have, however, is that the cooling processes cannot provide enough cooling to counter the warming. If you look at the past decade, the current bump and wiggle, we have to conclude that we have not fully represented all of the cooling mechanisms well enough to explain the observations. No doubt, we also have some details left in the calculation of the details of the warming. It does not seem smart to me to rely on self healing. (And remember ocean acidification.)
There is another thing the Solomon et al. paper makes me think about. They isolate a signal due to a change that is very small. Small change, big impact. This brings us back to a very important point thinking about climate change and humans sitting in the middle of it all. We sit in some sort of balance, and it often does not take something that is in an absolute sense large to cause a big change in the balance. Global warming is caused by a small change in radiative forcing due to large changes in carbon dioxide, which is, in fact, a gas present in relatively small quantities.
Trace substances matter a lot. If that does not make intuitive sense, remember, the amount of oil in the Gulf remains small compared with the amount of water. Pulling for topkill
r Bumps and Wiggles (1): Predictions and Projections Bumps and Wiggles (2): Some Jobs for Models and Modelers (Sun and Ocean) Bumps and Wiggles (3): Simple Earth Bumps and Wiggles (4): Volcanoes and Long Cycles Bumps and Wiggles (5): Still Following the Heat And here is Faceted Search of Blogs at climateknowledge.org