We could say clouds are too complicated for climate science – and ignore them forever. We could also just try to incorporate them in the models. If you do, chance is you’ll find climate sensitivity is underestimated, a very interesting publication in Science from 2012 tells us:
Anthropogenic climate change is in essence extra energy absorption in the atmosphere that therefore becomes more dynamic. Due to increased tropical convection the Hadley Cell intensifies and expands. On average the tropics become wetter, with increased (cumulus) clouds and precipitation, whereas the subtropics (at altitude – see blue area) become drier, with decreased (stratus) clouds. Both tropical and subtropical cloud changes might form an amplifying feedback to atmospheric warming – thereby increasing climate sensitivity. [This article is about the subtropical decrease of cooling clouds. If you're interested in the other warming cloud feedback, we also have an article about the increase of tropical warming clouds – yes, it's very complicated. We only try to help.]
Climate sensitivity higher due to Hadley Cell expansion? [If you want to learn more about climate changes to the Hadley Cell and other parts of Earth's general circulation – read our special article about 'Atlantic hurricanes and El Niño' (or just click the image above).]
The range of climate sensitivity estimates
At the very core of the climate science debate lies ‘Equilibrium Climate Sensitivity’ (ECS), the most likely atmospheric warming caused by a doubling of CO2, once climate inertia is overcome. You don’t reach the coupled warming values once the Keeling curve hits 400, 450, 500, etc ppm – you hit is decades later – the reason 99% of people (still) underestimate climatic urgency.
To make matters worse, there is a rather large band of uncertainty surrounding the expected ECS value. IPCC AR5 (2014) draws a ‘likely’ range (66% chance) that climate sensitivity is between 1.5 and 4.5 degrees Celsius – with most likely value around 3 degrees.
Climate model calculations of climate sensitivity show a ‘right-skewed distribution’ NASA explains their above graph. This suggests the probability of very large increases in temperature is greater than the probability of very small increases. [Below 2 degrees estimates for climate sensitivity, you're likely to enter the field of cherry picking under flawed climate forcing assumptions – corrupted science essentially.]
“Many models, particularly those with low climate sensitivity, fail to adequately resolve [subtropical] teleconnections and hence are identifiably biased” – Science, 2012
Now what is interesting to note is that climate models – although they offer a range of outcomes – see NASA image above, tend to show lower values for climate sensitivity than climate sensitivity estimates based on paleoclimatology, possibly because climate model climate sensitivity does not include (slow-running) carbon feedbacks (although live observations show these feedbacks are very real).
But a 2012 publication in Science by climatologists John Fasullo and Kevin Trenberth of the National Center for Atmospheric Research (NCAR – a cooperation programme of American universities) suggests the model calculation of climate sensitivity may ignore yet another important climate warming feedback: subtropical clouds.
Increased subtropical air subsidence: not just more droughts, also hotter
We’ve written al lot about increasing subtropical air subsidence – also this week, in our special article about record East-Mediterranean droughts. Due to climate change the Hadley Cell intensifies: More air moves up in the tropics (so more energy and rain in the Intertropical Convergance Zon (ITCZ – monsoon) and more air moves down in the subtropics, that also expand in poleward direction.
The subtropics are of course a zone of dominant high pressure systems – therefore low cloud cover and low precipitation. But there is some cloud cover on average, as there is some rain – which are both likely to decline as the atmosphere fills with heat-absorbing CO2.
Now what happens to your global climatic warming forecasts when you include this change in subtropical cloud cover in your climate model? Well, you can guess it based on the type of clouds – in zones of high pressure the dominant cloud type are the stratus family – thin, layered clouds, that absorb far less energy than cumulus clouds, and can dramatically increase albedo, therefore act as climate coolers. A drying of the atmosphere – that the researchers note – takes place in the subtropical subsidence zone (the 30 degrees latitude) but expands towards the 30-45 degrees latitude – Earth’s Meditteranean climates, where their model suggests net cloud cover would actually decrease most (see dotted line in first image in this article, at top) – most notably around 500 hPa (roughly translating to a height of around 5 kilometers of altitude in the troposphere) decreasing albedo and increasing solar heat absorption, therefore net climate warming.
Decreases in clouds and relative humidity are most notably modelled for the Mediterranean climate zones.
“Major questions persist. These include the relative contributions of various cloud types to the overall cloud feedback and the sources of biases in the vertical RH [relative humidity] and cloud distributions, and these are the focus of ongoing research. In a broader context, improved representation of regions of strong subsidence, particularly at low latitudes, is of fundamental importance. Such an improvement is essential not only for correctly simulating climate sensitivity, but also for characterizing changes in climate extremes and related impacts. Their scrutiny is therefore likely to be beneficial in understanding the broad range of uncertainties that currently exist in our future climate,” the authors conclude.
- Further reading: Expanding Hadley Cell do to climate change may create two warming cloud feedbacks. A decrease in cooling (stratus) subtropical clouds, and an increase in warming (cumulus) tropical clouds. If modelled, both cloud change factors could increase estimates for climate sensitivity.
© Rolf Schuttenhelm | www.bitsofscience.org