Mapping climate change

The impact of climatic change on global agriculture is an area of concern for food security, policy, investment etc. While agricultural productivity continues to increase, climatic change introduces an unwelcome element of uncertainty and risk.

David Lobell (Stanford University) and co-workers have related changes in crop yields to observed climatic shifts over the past thirty years. They found that long-term temperature changes are having a greater impact on yields in major agricultural areas than shifts in rainfall patterns. A grain trader might find this fact surprising. There is a well-known rule of thumb that crops benefit from plenty of rain during the vegetative (growth) stages and drier conditions during the fruiting/harvest stages. Low yields or poor quality occur if these conditions are not met. Therefore yield variability is often associated with rainfall variability. The finding by Lobell and co-workers is an example of how short-term variability can mask a longer-term underlying trend.

Nowadays climatic forecasts are made using state-of-the-art physics based models such as CFSv2. Where there is sufficient historical data, these models can also be used to recreate the state of the atmosphere in the past. The maps below show the trends in 2m temperature over land during 1982-2010. Trends were extracted from CFSv2 Reanalysis data using R‘s raster package and plotted using ggplot2 (“trends” means linear regression coefficients although there is markedly non-linearity in many places.)

The upper map is for December-January-February (DJF) and the lower for June-July-August (JJA). Globally, DJF warmed by ~0.36oC/decade over land while JJA warmed somewhat less ~0.28oC/decade. Winter in North America and Scandinavia warmed by several degrees during the period 1982-2010, whereas summer warming was much weaker. There are even some places where winter and summer trends are reversed. The US breadbasket states show little warming (even some cooling) during the summer  growing season, but this is far from the case in Central Europe or Russia.

Corresponding maps for rainfall look quite different:

 

While regional trends can be large ~ ±50mm/decade, there was no significant net wetting or drying trend over land.

The spatial and temporal structure of climatic shifts during 1982-2010 is much more complex than headline global warming numbers suggest. For agriculture, the details matter a great deal.

 

January 2, 2012   Posted in: Agriculture, Climate  No Comments

Cold Winter Sun?

Recent winters brought extreme cold to North West Europe and parts of the US. It is not likely that consecutive severe winters occurred purely by chance. If not, then what was the cause? And will it happen again this year?

Blocking and the Arctic Oscillation

Extreme conditions can occur when the normal variable circulation of the atmosphere gets stuck into a stationary “blocking” pattern. A block can extend over 1000km’s and persist for days or weeks. In 2010, the  block which brought a heat wave to Russia also caused floods in Pakistan.

Blocking anti-cyclones bring severe winter cold to NW Europe. Mild moist air is kept away and replaced by cold dry air. Under clear skies and with long winter nights, the land surface radiates away more heat than it receives. Lower and lower temperatures are reached and relief only comes when the blocking pattern breaks down.

The formation of Northern hemisphere winter blocks is associated with the negative phase of the Arctic Oscillation (AO). The AO index describes the strength of a ring or vortex of air which circulates around the pole, shown in (a) below. The origin of this westerly wind or “jet” is the North-South temperature gradient and the rotation of the earth (coriolis force). In winter, when the temperature gradient is large and the AO is positive, the jet helps confine very cold Arctic night air to polar latitudes.

 

 

A negative AO index means that the polar vortex is weaker than normal. In this case, the situation shown in (c) is likely to occur. The difference between high and low AO is analogous to an ice-skater who is very stable when spinning fast, but more likely to wobble and fall over when spinning slowly. The earth-scale meanderings of the jet stream are called Rossby waves. Larger amplitude meanderings such as (c) favour the formation of atmospheric blocks, which is why severe winter weather is associated with negative AO.

 

The above chart  of daily AO index since 1950 shows the recent period of negative index. The chart also shows that Arctic Oscillation is highly variable and irregular. Most of this derives from internal variability of the earth’s climate system. However researchers have known for some time that there is a connection between solar activity and AO e.g. the Little Ice Age (or Maunder Minimum ~1645-1715) was a period low solar activity and severe winters in NW Europe and the US. This has been a puzzle, because the brightness of the sun is nearly constant (variation ~ 0.1%).

Arctic Oscillation and Solar UV

 

 

Although only a small part of the sun’s output is in the ultra-violet (UV), variations in this part of the solar spectrum are now known to be fairly large (~8%).The above figure (from the UK Met Office) describes research linking the strength of the polar vortex to solar UV variability.[1] The proposed mechanism works as follows. Solar UV heats the stratosphere at ~ 25km where most of it is absorbed by ozone. Heating is greatest over the tropics where sunlight is most intense. Lower UV radiation means less heating, which means a lower North-South temperature gradient, which means weaker thermal winds in the stratosphere. If this effect is transmitted down to low levels (troposphere), it might weaken the entire polar vortex and push AO negative.

While this is an area of active research, it is probably a good idea to keep an eye on solar UV. Here is a plot of the total UV radation in the range 115-310nm using satellite data from SORCE. (This R script will download the latest data from SORCE and reproduce the plot.)

While total solar radiation has picked up this year, UV radiation is still weak. If the new research is right, the odds are shifted in favour of another severe winter.

 

 

[1] Solar forcing of winter climate variability in the Northern Hemisphere,  S. Ineson, A.Scaife, J. Knight, J. Manners, N. Dunstone, L. Gray, J. Haigh Nature Geoscience 4, 753–757 (2011)

 

November 8, 2011  Tags:   Posted in: Climate  No Comments