“The Weather Social” is a collaboration of weather and communication experts focused on narrowing the gap between scientific messages and society.
By Mike Nelson
Professor Scott Denning at Colorado State University is an atmospheric scientist and climate expert. He does an excellent job of taking complex subjects and breaking them down to explain the science in an easy to understand manner — no small feat!
I recently asked him to address some of the basics of the greenhouse effect and global warming.
Mike Nelson: Professor Denning, is the greenhouse effect and global warming something new, just in the past few decades?
Scott Denning: The greenhouse analogy is not new, it was first used by Joseph Fourier in the 1820s.
The absorption spectrum of carbon dioxide (CO2) was first measured by John Tyndall who also explained the role of CO2 in climate change in the 1860s.
The “degrees of warming per doubling of CO2” (logarithmic relationship) was first measured by Svante Arrhenius in the 1890’s. He also published his expectation that burning coal would cause global warming in 1896.
The increase in trapped heat from increased CO2 (3.7 Watts per square meter per doubling of CO2) is a 21st century number, based on the latest lab spectroscopy, satellite radiometry, and radiative transfer calculations.
Nelson: So, Arrhenius had this figured out nearly a century before global warming became a major news topic?
Denning: Yes, qualitatively this number is the same as Arrhenius, but he could never have calculated this so precisely.
So if you want to be precise, we have known about the role of CO2 absorption in warming based on lab measurements for 150 years, and we have known about the linear warming per doubling of CO2 for 120 years, but we’ve only had really precise data on the sensitivity of climate to CO2 in recent decades
Nelson: A quick clarification if you would not mind. So when we went from 1 ppm to 2 parts per million (ppm) of CO2, the heat retention increased 3.7 Watts per meter squared (W/m2) — the same for 2 ppm to 4 ppm, 4 to 8, etc. Now that we are at 400 ppm, it will take us up to 800 ppm for get the same effect, correct.
Denning: This is basically correct. Certainly 200 to 400 is the same as 400 to 800, 3.7 W/m2 based on best estimates. I’m not sure you can go all the way down to 1 to 2 or 2 to 4. The formula is dQ = 3.7 W/m2 * log(C2/C1)/log(2). I’m just not sure it can be extrapolated to such extremely low concentrations. But it hardly matters since the CO2 has never been that low.
Nelson: We know that positive feedbacks (increasing the warming) kick in such as the melting of the northern ice cap, increased water vapor, methane release due to melting permafrost. Are there negative feedbacks that counter this to any great degree?/strong>
Denning: The 3.7 W/m2 is from CO2 alone. Without feedbacks this would warm the Earth only 1.1°C (2°F).
The actual climate sensitivity is determined after this initial change (0.27°C per W/m2) gets amplified by positive feedbacks and also reduced by negative feedbacks.
Nelson: How do we know these feedbacks exist?
Denning: We know there must be negative feedbacks because Earth has almost always had liquid water throughout geologic time (oceans never froze solid or boiled away).
We know there must be positive feedbacks because climate has changed in the past even with very small changes to the radiation balance.
Nelson: What are some of the most important feedbacks?
Denning: The most important positive feedbacks are:
Water vapor (warmer oceans increase evaporation, water vapor is a powerful GHG)
Ice and snow albedo (melting snow increases absorption of solar radiation)
High clouds (extra water vapor condenses to make high clouds that let sun in but block outgoing LW (longwave radiation) )
The most important negative feedbacks are:
Increased radiative cooling (Stefan-Boltzmann relationship, OLR = sigma * T^4) Do not worry, you won’t be tested!
Surface warming and extra moisture promote convective clouds that transport heat to upper troposphere where it can be more efficiently radiated to space
Low clouds (extra water vapor condenses to make low clouds that block sun but emit a lot of longwave (Earth) radiation upward because they’re warm — near the ground)
Nelson: What are the most difficult challenges in understanding the various feedbacks?
Denning: The hardest ones to quantify are clouds, because low clouds cool the climate but high clouds warm the climate. All clouds have conflicting effects on surface temperatures. They reflect sunlight (shortwave radiation) to space so cool the surface. In the longwave radiation, they emit to space at a temperature that’s colder than the surface, so act to retain heat.
Low clouds are loaded with liquid water droplets so they are very bright when seen from above, but because they are low they are relatively warm so they emit almost as much longwave radiation up as the surface. Therefore low clouds act to cool the climate.
Nelson: What impact do mid and high level clouds have on our climate?
Denning: High clouds (think cirrus or cirro-stratus or even altostratus) have much less liquid water or are made entirely of ice. Thin, diaphanous clouds let most of the sunlight right through. But in the longwave they are almost perfect blackbodies (good at radiating heat).
They emit to space at very cold upper tropospheric temperatures, so the Earth loses much less energy to space than if the surface could emit directly. Therefore the longwave effect “wins” and high clouds act to warm the climate.
Nelson: Are we very good at modeling clouds in terms of climate change?
Denning: Predicting the net effect of changes in clouds in a warming climate depend very sensitively on what kinds of clouds form from all that extra water vapor that evaporates from the warmer oceans.
Feedbacks are complicated. We can’t just decide what will happen based on simple logical arguments. We need to either crank through the brutal arithmetic of full-blown climate models, or look at the total climate sensitivity by studying past climate change.
Nelson: So do the climate models and past climate studies agree?
Denning: Luckily, these two independent lines of research lead to nearly identical conclusions: overall climate sensitivity of about 0.8 degree Celsius per Watt/m2 of extra radiation. In other words, about 3 Celsius global warming per doubling of CO2.
Nelson: What are your thoughts on a tipping point. I hear that mentioned by author Bill McKibben. He talks of 350 ppm, is it already too late?
Denning: There are certainly potential tipping points: for example the rapid collapse of ice sheets, melting permafrost, die-back of tropical forests, shutdown of thermohaline circulation. Any of these would make the climate warm much more than predicted by paleo data or climate models. There may be others that we haven’t thought about.
Nelson: Thank you for all of the physical science, do you have thoughts on the political science of global warming?
Denning: In my opinion many of the problems of rapid global warming are probably avoidable if we adopt ambitious measures to develop non-carbon energy before China, India, and Africa emerge as major energy users in coming decades.
Certainly the likelihood of getting bitten by one or more of these tipping points is far greater the longer we wait to deploy solutions.
In my opinion both the 350 ppm goal and the “it’s too late” argument are self-defeating. 450 ppm is better than 500 ppm, and 500 ppm is better than 1000 ppm. It’s never too late to stop making it worse!
Professor Denning, thank you for your time!
This originally appeared on The Weather Social.
