The Earth sports two large belts of predominantly dry, sunny weather – the subtropical dry zones.
We’ll explain how and why these desert climate zones are advancing.
One’s in the Northern Hemisphere and spans from the subtropical North Atlantic ocean to the hot deserts of central Asia. The other stretches from southern Africa’s deserts to the desolate Australian outback.
Not all areas they cover have harsh conditions. If you’ve visited the Azores Islands, or the Mediterranean, you’ve had first-hand experience. Those places benefit from their adjacent or surrounding seas or oceans (moisture!). Sadly, they are generally much smaller (and busier!) than the major deserts.
‘The Earth sports two large belts of predominantly dry, sunny weather – the subtropical dry zones. We’ll explain how and why these desert climate zones are advancing.’
The location of these zones is governed by the strength and shape of a world-spanning circulation system – one that also helps sustain the temperate, highly hospitable climates of many mid-latitude locations – in particular, large parts of the USA and Europe.
It exists because of an unending battle against imbalance in Earth’s atmosphere…
The Atmosphere Abhors Inequality
The Earth’s climate is constantly being forced away from a balanced status.
The laws of physics would rather it has the same atmospheric conditions everywhere, but that pesky sun keeps on heating the low latitudes (towards equator) more than the high ones (towards poles), throwing a glaring spanner in the works.
To counter this, the atmosphere has developed vast circulations. These operate horizontally and vertically to transport heat poleward, and cold air equatorward.
It could have been one big equator-pole circulation, but air isn’t the best at retaining its temperature as it travels, so complications arise.
Triple Turnover: The Hadley, Ferrel and Polar Cells
Here’s a quick overview how these circulations are currently arranged (Figure 1)… or at least have been until recently. More on that later.
Figure 1: Diagram depicting the three major circulations that we currently observe in Earth’s atmospheric system. Their latitudinal positions are as was typical of the 2nd half of the 20th Century (and probably long before then). Note that the equator situation is partially cloudy, with the rain mostly falling in very heavy showers or thunderstorms.
(If you’re wondering why these circulations exist in this configuration, well, that’s a very complicated matter that would require whole lectures to explain in good detail! For this blog entry, we’ll have to suffice with ‘it is what it is’, so that we can focus on how it’s changing and the consequences of that.)
Changing Behaviour: A Difficult Diagnosis
There are numerous scientific studies that have identified significant changes in the Hadley Cell, with knock-on effects on the Ferrel Cell. This has been done using a variety of different measures, from mass transports to rainfall-to-evaporation ratios.
Unfortunately, I don’t have easy access to all the required data for those measures. However, for a general overview, I believe sea-level pressure (SLP) can give some sense of what’s going on (Figure 2).
Figure 2: Map displaying the mean decadal trend in mean sea-level pressure (mb) 1970s-2010s. The dashed lines indicate the typical position of the subtropical highs and mid-latitude lows during the 20th Century. The lettered arrows indicate the major circulations: H = Hadley; F = Ferrel; P = Polar.
It quickly stands out that – with the (intriguing…) exception of Africa – the positive trends are grouped into two main belts. They’re both roughly parallel to the subtropical highs lines in each hemisphere.
The Southern one is by far the most consistent, which is likely down to the relatively open, flat surfaces there. The Northern hemisphere counterpart is distorted by numerous mountain ranges, which the major airflows must navigate.
The overall picture is one of positive SLP trends poleward of the historical (20th Century) average position of the subtropical highs. This can be linked to either a poleward shift or an expansion of the descending part of the Hadley Cell.
‘What’s happening is essentially a poleward advance of the desert climate zones. The Sahara is encroaching upon southern Europe via the Mediterranean. The Australian deserts are spreading toward the highly populated southern coasts.’
This has serious implications for those located beneath the positive SLP trends.
The subtropical highs not only bring predominantly dry weather, but also warm (or hot), subtropical air. For land areas in winter, that means strong reductions in snowfall, days with ice and heating demands. In summer, it increases the frequency, duration and intensity of heatwaves and droughts. A few spots may see a reduction in yearly total rainfall, too*.
We’ve seen prominent examples of these impacts in recent years, such as Europe’s record-hot summer of 2018 and exceptionally mild, low-snow Dec-Jan of 2019-20. Then there’s Australia’s dry winter-spring of 2019 followed by a record-breaking Dec-Jan 2019-20 heatwave. The consequences were all over the news.
What’s happening is essentially a poleward advance of the desert climate zones. The Sahara is encroaching upon southern Europe via the Mediterranean. The Australian deserts are spreading toward the highly populated southern coasts.
Slowly but Seriously
Via analysis of multiple reanalysis datasets, Hu, Huang & Zhou (2018) identified the average rate of Hadley Cell expansion to be approx. 1° of latitude per decade. That’s about 111 km, or 69 miles.
That may not sound like much for a whole decade, but it adds up… and for all we know, the pace may pick up in the decades to come. Model simulations show a lot of variation depending on how much weighting is given to differing forcing factors.
Counteraction Seeds Hope
Regardless, all is not lost: we can fight back. In some countries, the advance of deserts is being combatted through initiatives to plant millions of acres of protective forest (thanks to $ billions of funding!) or revise land-use practices, for example.
It remains to be seen just how effective these projects are, but there are reasons to be hopeful.
Through observation, we know that plants release water vapour during photosynthesis. In this way, large areas of vegetation can increase cloud and rainfall frequency. Not only that, but large plants shade the ground, keeping it from drying out as quickly after rainfall. The hydrological cycle can be transformed into one far more sustainable for living organisms.
* There is a counterbalancing climate trend when it comes to how much rain falls in a year. As the climate warms, the average capacity of air to contain moisture increases – by about 7% for every 1°C. This means that when it does rain, it can rain harder.