You are a student of an agricultural discipline and suddenly you are asked a question about climate change. ‘It’s going to get warmer in general,’ you start to say, ‘more variable with bigger storms and droughts.’

You sound confident, but deep down you know that you’re not so sure about what will happen to pesticides. Will everything be better in the future? You need some hard science so you can make up your own mind - with so little time, where will you find that science? Help is at hand!

Here are five facts to give you all you need, to sound like  the expert that you want to be.

1 - Pesticides won’t always degrade faster if it’s warmer

Chemical reactions do go faster at warm temperatures. So you would expect that pesticides would degrade faster in Spain than in Sweden.

However, hot temperatures are often linked to dry soils. Microbial reaction rates in dry soils are slower than in wet soils, although pure chemical reactions don’t have this limitation.

 

 

Temperature and moisture maps of Europe. Images: Atlas of the biosphere

The rates in Spain are four times faster due to temperature but five times slower due to moisture. The reaction rates may actually be faster in Sweden! You have to consider both temperature and moisture when you think about pesticide reaction rates in soil.

2 - Warmer water doesn’t necessarily contain more pesticide

Substances tend to get more volatile, the more that you heat them up. If you place a large body of water, like a lake, in a slightly warmer climate, the amount of pesticide in it will decrease slightly.

At a fixed temperature and pressure, there is a constant ratio of concentrations in the air and water phases. But change the temperature and the ratio changes too, and more of the substance ends up in the air phase.  

Losses to the air are really small for most pesticides, and contamination of water is usually a bigger concern. But it is still interesting to note that pesticides tend to leave hotter water and head up, up and away into the atmosphere.

3 - Cold weather can reduce pesticide leaching 

Leaching is the process when water that trickles through the soil takes pesticide with it, on a journey that can end at the water table. In Europe, colder weather is often associated with wet weather, so you would normally expect the excess rain to carry the pesticide on its journey all the more. However, there are situations where this would be false.

 

Pesticide leaching has paused. Image: Richard Walker/ Flickr

1. Really cold weather    

The ground freezes and nothing moves anywhere. When things start to warm up a bit, there could be a fair abundance of water trying to percolate through. The question is; will the ground stay frozen long enough so that the water runs off before it ever gets into the soil?

2. Cold weather can pull pesticide out of the water

The amount of pesticide in the water is balanced with the pesticide stuck to the soil – we usually call this sticking ‘sorption’. Several studies indicate that when the temperature drops, the balance swings away from the water and towards the solid. That means there is less available for leaching.

4 - A lot of rain doesn’t mean a lot of pesticide runoff 

If you have heavy rain, you have several different factors. 

First of all, the state of the soil has an impact – is it easy for the water to penetrate? 

You also have the slope: steeply sloping land will lead to runoff earlier in the rain event than gently sloping land. 

Finally, you have the pattern of rainfall. If it all falls at once, runoff will be much more likely, because there won’t be time for it to soak in.  

The pattern of rainfall intensity is called the ‘erosivity’ of the rain. If you take the average erosivity over a long period of time, you can build a map of where erosivity is generally highest. 

 

There are some pretty wet places with low erosivity, such as Ireland, Denmark and Northern Germany. Some dry spots in Spain and southern Italy have high erosivity. Total rainfall (left) and erosivity (right). Image: Panagos et al. (2015)

5 - The main effects of climate change on pesticide fate will not be due to physics or chemistry.

There is a clear consensus that the climate is changing. The climate influences a range of agro-ecological features, for example the timing of pest infestation or infection, the rate of plant growth and the soil conditions, such as the organic carbon content, moisture status and the extent of cracking.

 

Would you choose to sow oilseed rape? It partly depends on the climate. Image: Simon Rowe/Flickr

The indirect effects of climate on pesticide fate can be considered as a tension between the twin societal drivers to maximise production while maintaining environmental safety. Will the indirect effects outweigh the direct effects? I think they will and I am not alone.

So, the biggest effect of climate change on pesticide fate is not physics and chemistry but a series of responses of farmers, consumers, producers, retailers and politicians in  how we all decide to react to the changes.