Desertification and Population Pressure

My last post on climate change evidence concerned the doubt that still surrounds the issue of man-caused global warming. Personally, I can't see how we could possibly not have influenced the climate – we can’t even brush out teeth without damaging consequences.

That’s not necessarily an exaggeration, either. Desertification of farmland in semi-arid areas poses an increasingly ominous threat to the wellbeing of both our environment and the human race. UNEP reckon about 20 million hectares of land are lost to desertification each year. That’s around 200,000 km² or a little 1% of the world’s estimated arable land. It doesn’t take a genius to realize that isn’t going to leave us with much in around 100 years or so.

You might be tempted to think that this is a strictly developing world phenomenon, but you'd be wrong. In fact the most rapid desertification is happening in Australia, Spain and the USA. What’s causing it? Like I say: brushing your teeth.

Causes of Desertification

Okay, so it’s not quite that simple. Our ecosystem is both fragile and complex and any number of things can tip it out of balance. The main cause of desertification is poor soil conservation. The top layer of any soil is important and failing to maintain a healthy layer of humus is one of the prime reasons for the failure of arable land.

Crop rotation, composting and responsible use of fertilizer are all ways of getting the best out of soil but many farms practice intensive monoculture and use excessive chemical inputs to get the most out of the land. This inevitably leads to short term profit and long term soil degradation. Often such practices come about because of western business models that emphasis profit over responsibility. The gluttony of the developed world could well be contributing to the slow death of developing countries with marginal farmland.

Deforestation is another direct contributor. Reducing the concentration of trees affects the thermal properties of the land and often leads to reduced rainfall. Without an adequate water supply, the humus layer begins to degrade rather quickly and the soil begins to thin and die.

In developed countries with high urban densities and efficient water systems this same effect can be produced by excessive water consumption. Tapping into the aquifer and draining the water table can intensify the effects of dry periods and cause degradation on the surface. Excessive water demands from urbanization and tourism in countries such as Spain are putting large areas of farmland in danger – the UN reckons around 12% of Europe is in immediate danger of desertification and as much as 60% on the Iberian peninsula.

Which brings me back to brushing your teeth. You can use 12 gallons on water when you run a tap for 3 minutes - switching the tap off while you brush can save a lot of water over the year. There's a lot more you could be doing, too - taking shorter showers and showering more often than bathing, for example. But the toothbrushing fact is a nice clean tagline which media men (and me) like to seize upon. I guess it emphasises just how important some of our most banal actions are. Water conservation isn’t just a key issue in Sub-Saharan Africa; it’s an important principle that should apply to everybody and which needs to be taken seriously.

Feedback from Population Growth

It gets worse, too. Because of the impact of intensive farming and urbanization on the rate of desertification, the rate of damage theoretically increases as a population grows. The UN estimated the Earth’s population to be around 6.8 billion at the end of 2009 and the current rate of increase is around 100 million per year.

An optimistic blog post by Fat Knowledge estimates the ultimate carrying capacity of the Earth to be anything from 30 billion to 100 billion people – if all available farmland were used and everybody ate corn. But this estimation does nothing to take account of the effect that population growth will have on available farmland.

The agricultural capacity of the planet is already in decline and will continue to trend that way as the population grows. Sooner or later we’re not only going to be in a sticky situation; we'll have more people than the planet can support and our ability to support them is going to go into rapid decline. If Thomas Malthus is to be believed we'll probably keep on breeding past this point too, until something cataclysmic happens.

There are examples of this from history. History tells us that the pre-renaissance population crash in the UK was the result of the Black Death, but such a simplistic estimation fails to recognise that the population was already in steep decline before plague hit. The Black Death certainly helped, a large part of the near 3 million reduction in population was the result of crop failure.

I’m not going to try and translate that into a global context. The mesasge however if fairly clear; desertification is a very real threat and sooner or later the degradation of farmland coupled with excessive population growth is going to lead to a major and rather terrible global food crisis.

Not that I want to ruin your morning or anything.

2000 Years of Climate Change Evidence

The scandal over ‘Climategate’ has got a lot of people rethinking their stance on global warming; at least, it has lent fuel to the arguments of those who never wanted to believe it anyway. Most of us have to admit, too, that we’re incompetent to assess the information available. After all, if the issue is proving so troublesome for our top scientists then what chance to do we have?

One of the main problems scientists have is filtering through the available data for meaningful information. There’s a lot of ‘noise’ and a lot of bad science out there and modeling climate is such a complex task that it’s still impossible to predict anything with a degree of certainty.

Still – what if it really was all a myth? What if human influences on climate change were just a bad dream after all?

Historical Climate Change Records

Whether or not climate change occurs is not the issue. We know that it does and that it has been happening for million years. That is the issue. It’s been happening long before humans showed up so who can really say for sure that recent rises in temperatures have been down to us?

Most famous studies of climate variation over the last few millennia show a marked increase in global averages from the end of the 19^th century. For example, the diagram below is borrowed from a 2001 report by the Intergovernmental Panel on Climate Change.

The picture appears to show a stark rise in global temperature through the twentieth century and has been used as clear evidence of anthropogenic global warming. But a growing number of studies have challenged this model. The next diagram is reproduced from a study by Dr Craig Loehle, a professor of mathematics and ecology, who compiled the global average temperature over the last 2000 years using 18 different data sets – without using tree ring data.

The trends in this diagram are markedly different. One of the main reasons for this is that conventional temperature data is often calculated using dendrochronology but this method can be flawed, as trees react to a number of environmental factors – not just temperature – and may also adapt to changing climate, adding a feedback affect to any results.

In Loehle’s model, temperatures have changed a great deal in the last 2000 years and current temperatures aren’t actually any hotter than they were in 1000AD. There is a clear upward trend from around the start of the Industrial Revolution but we can’t necessarily attribute that to human influences because it’s all happened before.

Assessing Historic Temperature Data

There are potentially a number of problems with both sets of data.

Obtaining historical temperature data isn’t easy, as thermometer records have only existed since the 19^th century. There are various techniques for doing it, some more reliable than others. The data in the first diagram includes tree-ring studies that may not have been accurate, while the grey shadow behind the line of best fit indicates just how much doubt there is over the precision of the observed results.

But Loehle’s study doesn’t include nearly enough data to give an accurate global picture. All 18 data sets are from sources by the ocean; 15 of the 18 data sets are from the northern hemisphere; half of the data sets are from a ring around the North Atlantic. The geographical bias indicates that there may well have been correlation between some sets due to local rather than global climate trends.

Even if the 18 data sets aren’t locally correlated, 18 sets of data is a small enough number that even random changes could appear to show trends. Some of the data sets had a granularity of 100 years. Before we even begin to factor in calculation error there’s a lot of room for interpretation and interpolation.

I set up an excel spreadsheet to generate random temperature trends, with maximum 100 year changes of 1C, and employed averaging techniques at points separated by 20 years. A representative sample of the results is shown below.

While the variance in these graphs isn't quite as high as that in Loehle's data, it does shows that 18 limited, independent sets could quite easily have shown apparent trends, such as the highs and lows produced by the Medieval Warm Period and the Little Ice Age, even if there was no real trend and the temperatures were fluctuating independently and randomly. Local changes in climate at each of these points do not necessarily point to a global trend.

The Truth about Global Warming

The point of all this has been show that our knowledge of past climate change is still patchy and that the evidence for claiming ‘this has all happened before’ is just as patchy as the evidence showing that temperature rise in the 20^th century has been unique.

And even if the statistics were accurate they tell us nothing about the causes. There are many possible natural influences on climate change – changes in the solar output, changes in the Earth’s orbit, continental drift, volcanic activity, shifts in sea currents – and a number of secondary feedback processes, such as changing albedo from ice and cloud cover, which can serve as amplifiers.

But it’s also possible that atmospheric composition, as influenced by humans, could do the same. Regardless of whether it was caused by man or is part of a natural cycle, there was a definite upswing in temperature in the 20th century. We have far more data about the last 100 years than any other period before it, enough to show global temperature changes with an accuracy that can’t be achieved from a few sporadic data sets. What we lack is knowledge of the mechanisms governing the change.

So the truth about global warming? It's real. But we don't know for sure if it's down to humans, and we probably won't until it's too late.