Predicting the future: climate change modelling requires cautious use

A new paper published in Nature Climate Change (Harrison et al. Implications of evaluation of CMIP5 palaeosimulations for climate projections. Nature Climate Change 5: 735-743. August 2015) suggests that we need to exercise considerable caution about future projections of regional climate changes.

The paper looks at features of the climate that are characteristic of 21st century projections and then examines whether state-of-the-art climate models predict these features correctly in the geologic past. Lead author, Sandy Harrison says “This is necessary because future climate changes will be much larger than anything we have experienced in last hundred years or so when we have meteorological observations. But there is abundant geologic evidence for large climate changes in the past that can be used to see whether the models are working”.

The paper shows that climate models capture the large-scale patterns of temperature change, including the fact that warming over land is more than twice as much as over the ocean and that the biggest warming will occur in high latitudes. They also reproduced the observed global relationship between precipitation changes and temperature as temperature increases.

However, the paper shows that the models do not capture the scale of regional climate changes. The models predict an increase in monsoon rainfall both in the future projections and in the mid-Holocene, 6000 years ago, in response to enhanced land-sea temperature contrast. Abundant evidence shows that the Sahara desert was vegetated and supported abundant wildlife during the mid-Holocene, but models underestimate the observed change in precipitation in northern Africa by at least 50%.

Modelled changes can also be opposite to what actually occurred.  Models predict drying in the mid-continents in both the future and the mid-Holocene. The mid-Holocene predictions for central Eurasia are wrong – palaeoenvironmental data show that this region was in fact wetter and cooler than today – and this raises serious concerns about the future projections for the region.

Our evaluations give us confidence that the general trajectory of modelled global warming is correct and that means that model estimates of what we need to do to limit global warming, say to less than two degrees, are likely to be realistic – which is very good news indeed. However, many government agencies want to use the projections for planning purposes at a local scale and here I think we have to exercise considerable caution about what the models say”.

Mountains warming faster, say scientists

An international team of scientists is calling for urgent and rigorous monitoring of temperature patterns in mountain regions after finding evidence that high elevations could be warming faster than previously thought.

The research team says that without substantially better information, we risk underestimating the severity of a number of already looming problems, including water shortages and the possible extinction of some alpine flora and fauna.

The research is published today in the journal Nature Climate Change.

Caucasus Mountains, Russia

Caucasus Mountains, Russia

Co-author Dr Maria Shahgedanova, University of Reading, said: “The evidence that mountains are warming faster than low elevations is growing but we still lack detailed information from both observations and models and, as a result, cannot reliably assess impact of the high-elevation warming. These can potentially affect not only high-altitude ecosystems but also water supply from snow and glacier ice and hazards associated with shrinking cryosphere which will impact population at lower elevations.

“To address the issue of elevation-dependent warming and its impacts, we need to employ a combination of automated ground-based measurement networks in different mountainous systems, high-resolution models and satellite imagery. Scientists from the University of Reading work on the development of all three components in the mountainous regions of Europe and Asia.”

The most striking evidence that mountain regions are warming more rapidly than surrounding regions comes from the Tibetan plateau. Here temperatures have risen steadily over the past 50 years and the rate of change is speeding up. But masked by this general climate warming are pronounced differences at different elevations. For example, over the past 20 years temperatures above 4,000 m have warmed nearly 75 per cent faster than temperatures in areas below 2,000 m.

The team of scientists came together as part of the Mountain Research Initiative, a mountain global change research effort funded by the Swiss National Foundation. The team includes scientists from the UK, US, Switzerland, Canada, Ecuador, Pakistan, China, Italy, Austria and Kazakhstan. Between them, they have studied data on mountain temperatures worldwide collected over the past 60-70 years.

Lead author, Dr Nick Pepin, of the University of Portsmouth said: “Most current predictions are based on incomplete and imperfect data, but if we are right and mountains are warming more rapidly than other environments, the social and economic consequences could be serious, and we could see much more dramatic changes much sooner than previously thought.”

Improved observations, satellite-based remote sensing and climate model simulations are all needed to gain a true picture of warming in mountain regions, the researchers say. Much of that requires international agreement and collaboration – and funding.

Among the reasons the researchers examined for faster rates of temperature increase in mountain regions are:

–          Loss of snow and ice, leading to more exposed land surface at high elevation warming up faster in the sun;

–          Increasing release of heat in the high atmosphere. A warmer atmosphere holds more moisture, which, when condensing as clouds at high elevation, releases more heat to the mountain environment;

–          Aerosol pollutants at low elevations, including haze, dust and smoke, reduces  warming at those elevations, thus increasing the difference in rates of warming between low and high elevations;

–          Dust and soot deposited on the surface at high elevations causes more incoming sunlight to be converted to heat;

–          The complex combination of any or all of the above factors in different regions and at different times of the year.

Records of weather patterns at high altitudes are ‘extremely sparse’, the researchers found. The density of weather stations above 4,500 m is roughly one-tenth that in areas below that elevation. Long-term data, crucial for detecting patterns, doesn’t yet exist above 5,000 m anywhere in the world. The longest observations above this elevation are 10 years on the summit of Kilimanjaro.


Read more about Dr Shahgedanova at her staff profile.