Drought – worse than we thought

Inconsistencies with drought models that don’t account for sea surface temperature changes mean that drought in a climate changed world could be worse than predicted.

WHO: Aiguo Dai, National Center for Atmospheric Research, Boulder, Colorado, USA

WHAT: Looking at the impact of sea surface temperature variability on drought

WHEN: January 2013

WHERE: Nature Climate Change, Vol. 3, No. 1, January 2013

TITLE: Increasing drought under global warming in observations and models (open access)

Climate deniers love it when the models are slightly wrong for predicting future climate changes, and believe me, I’d love it if climate change weren’t so verifiably real and we could all retire and live la dolce vita.

However, that’s not reality, and in the case of this paper, where the model doesn’t quite line up with the observed changes that’s because it’s worse than we previously though. Oh dear.

Global warming since the 1980s has contributed to an 8% increase in drought-ridden areas in the 2000s. It’s led to things like diminished corn crops and the steady draining of underground water aquifers in the USA, much of which is currently experiencing persistent drought. The letter L on the map below stands for long term drought.

Long term drought in the Southwest of the USA (from US Drought Monitor)

Long term drought in the Southwest of the USA (from US Drought Monitor)

What’s that got to do with climate models? Well, while the droughts in Southern Europe or my homeland of Australia are due to lack of rain drying things out, drought can also be from increased evaporation from warmer air temperatures, which the models don’t fully take into account.

These droughts are harder to measure because they’re related to sea surface temperature changes that take decades and can be hard to identify as a human forced signal rather than just natural variations. So this researcher compared sea surface temperatures with drought predictions and observed warming to try and work out what is going on.

Predicted changes in soil moisture globally for 1980–2080 (black dots are where 9 out of 11 models agree on data) (from paper)

Predicted changes in soil moisture globally for 1980–2080 (black dots are where 9 out of 11 models agree on data) (from paper)

There were two areas where the models differed from the observed changes – the Sahel area in Africa and the USA.

In the Sahel, the models predicted there would be warming in the North Atlantic Ocean which would lead to increased rain. What actually happened was that there was large warming in the South Atlantic Ocean compared to the North Atlantic and steady warming over the Indian Ocean which meant less rain, not more. Similarly, for the predicted patterns in the USA, the influence of the Pacific Multidecadal Oscillation was not known to be influenced by human climate forcing. However, it switched to a warm phase from above-normal sea surface temperature.

Top: Observed sea surface temperatures. Bottom: predicted sea surface temperatures (from paper)

Top: Observed sea surface temperatures. Bottom: predicted sea surface temperatures (from paper)

These sea surface variations that were missed in some of the previous models have some obvious consequences for planning for the slow pressure cooker of stress that drought is on anyone living through it, let alone trying to make a living from agriculture.

The researcher noted that there were also some differences from the models when looking at sulphate aerosols, however for the 21st Century the signal from greenhouse gases will be much stronger than those from aerosols, so shouldn’t mess with the data too much.

So what does this all mean? Well, it means that there are both regional and broader trends for drought in a changed climate. The broad patterns are fairly stable ‘because of the large forced trend compared with natural variations’, which is scientist for humans are making a large enough mess out of this to see the evidence clearly.

The paper ends quite bluntly stating that having re-worked the simulations to take into account the new data for sea surface temperature and other variables, that it’s only more bad news.

It’s likely to be ‘severe drought conditions by the late half of this century over many densely populated areas such as Europe, the eastern USA, southeast Asia and Brazil. This dire prediction could have devastating impacts on a large number of the population if the model’s regional predictions turn out to be true.’

Yes, a researcher actually used the word ‘dire’ in a scientific paper. Oh, and this was with an intermediate emissions scenario, not the business as usual path we’re currently all on. How about we all agree to stop burning carbon now?

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When the Party’s Over: Permian Mass Extinction

“The implication of our study is that elevated CO2 is sufficient to lead to inhospitable conditions for marine life and excessively high temperatures over land would contribute to the demise of terrestrial life.”

WHO: Jeffrey T. Kiehl, Christine A. Shields, Climate Change Research Section, National Center for Atmospheric Research, Boulder, Colorado, USA

WHAT: A complex climate model of atmospheric, ocean and land conditions at the Permian mass extinction 251 million years ago to look at CO2 concentrations and their effect.

WHEN: September 2005

WHERE: Geological Society of America, Geology vol. 33 no. 9, September 2005

TITLE: Climate simulation of the latest Permian: Implications for mass extinction

The largest mass extinction on earth occurred approximately 251million years ago at the end of the Permian geologic era. Almost 95% of all ocean species and 70% of land species died, and research has shown that what probably happened to cause this extinction was carbon dioxide levels.

As the saying goes; those who do not learn from history are doomed to repeat it, so let’s see what happened to the planet 251 million years ago and work out how we humans can avoid doing it to ourselves at high speed.

This research paper from 2005 did the first comprehensive climate model of the Permian extinction, which means their model was complicated enough to include the interaction between the land and the oceans (as different to ‘uncoupled’ models that just looked at one or the other and not how they affected each other).

The researchers used the CCSM3 climate model that is currently housed at the National Centre for Atmospheric Research (NCAR) and is one of the major climate models currently being used by the IPCC to look forward and model how our climate may change with increasing atmospheric carbon pollution (or emission reduction). They organised their model to have ‘realistic boundary conditions’ for things like ocean layers (25 ocean layers for those playing at home), atmospheric resolution and energy system balance. They then ran the simulation for 900 years with current conditions and matched it with observed atmospheric conditions and got all of their data points correct with observed data.

Then, they made their model Permian, which meant taking CO2 concentrations and increasing them from our current 397ppm to 3,550ppm which is the estimated CO2 concentrations from the end of the Permian era.

What did ramping up the CO2 in this manner do for the planet’s living conditions in the model? It increased the global average temperature to a very high 23.5oC (the historic global average temperature for the Holocene (current era) is 14oC).

Oceans
Changing the CO2 concentrations so dramatically in the model changed the global average ocean surface temperature 4oC warmer than current conditions. Looking at all the ocean layers in their model, the water was warmer in deeper areas as well, with some areas at depths of 3000m below sea level measuring 4.5-5oC where they are currently near freezing.

The greatest warming in the oceans occurred at higher latitudes, where ocean temps were modelled at 8oC warmer than present, while equatorial tropical oceans were not substantially warmer. The oceans were also much saltier than they currently are.

The big problem for all of the things that called the ocean home at the end of the Permian era is the slowing of ocean circulation and mixing. Currently, dense salty water cools at the poles and sinks, oxygenating and mixing with deeper water allowing complex organisms to grow and live. If this slows down, which it did in this model, it has serious consequences for all ocean residents.

Current ocean circulation patterns (NOAA, Wikimedia commons)

Current ocean circulation patterns (NOAA, Wikimedia commons)

Their Permian model measured ocean overturning circulation around 10 Sv (million cubic metres per second), whereas current ocean overturning circulation is around 15-23 Sv. The researchers think the ocean currents could have slowed down enough to create anoxic oceans, which are also known as ‘ocean dead zones’ or ‘Canfield Oceans’, and stated that it set the stage for a large-scale marine die off.

Land
If the end of the Permian was pretty nasty for ocean residents, how did it fare for land-dwellers? What happened to the tetrapods of Gondwanaland? Well it looked really different to how earth looks today.

Permian land mass (Wikimedia commons)

Permian land mass (Wikimedia commons)

There were deciduous forests at high latitudes, and the elevated CO2 in the model was the dominant reason for warm, ice free Polar Regions (which also hindered ocean circulation). Land surface temperatures were between 10 – 40oC warmer than they are today. In their model, dry sub-tropical climates like the Mediterranean or Los Angeles and Southern California were much hotter, with the average daily minimum temperatures around 51oC. Yes, Los Angeles, your overnight low could be 51oC.

Understandably, the authors state that ‘these extreme daily temperature maxima in these regions could contribute to a decrease in terrestrial flora and fauna’, which is scientist for ‘it’s so damn hot outside nothing except cacti can grow’.

All of these changes were run over a 2,700 year period in the model, which if you take the 2005 CO2 concentration of 379ppm as your base is an increase of 1.17ppm per year.

This is the important bit to remember if we’re going to learn from history and not go the way of the Permian residents. Our current rate of increase in CO2 concentrations is 2ppm per year, which means we are on a super speed path to mass extinction. If we continue with business as usual, which has been aptly renamed ‘business as suicide’ by climate blogger Joe Romm, we will be at the end of the next mass extinction in around 1,500 years.

Where humanity is headed (from Royal Society Publishing)

Where humanity is headed (from Royal Society Publishing)

All we need to do to guarantee this being the outcome of all of humanity is keep the status quo and keep burning fossil fuels and the entire sum of humans as a species on this planet will be a tiny geological blip where we turned up, became the most successful invasive species on the globe, burned everything in sight and kept burning it even when we knew it was killing us.

However, I think this part from the paper’s conclusion should give most of us a pause for thought;

 ‘Given the sensitivity of ocean circulation to high-latitude warming, it is hypothesized that some critical level of high-latitude warming was reached where connection of surface waters to the deep ocean was dramatically reduced, thus leading to a shutdown of marine biologic activity, which in turn would have led to increased atmospheric CO2 and accelerated warming.’

As a species, if we are going to survive we need to make sure we do not go past any of those critical levels of warming or tipping points. Which means we need to make sure we stop burning carbon as fast as possible. Otherwise, T-Rex outlasted us as a species by about two million years which would be kinda embarrassing.