By Prof. Antoine Bret

Climate change is news. However, news, web or the press, cannot always convey answers to some questions as frequent as concrete. These questions, which often come back in one form or another, are:
1. How can we talk about climate in 100 years if we do not know if it will rain in 20 days?
2. How do we know that there is a climate change?
3. How do we know it comes from the increase in greenhouse gases (GHG)?
4. How do we know that these GHGs come from human activity?
5. Do the experts agree?
I will try to answer them here as briefly as possible 1.
1. How can we talk about climate in 100 years….?
It is a very pertinent question indeed, which will allow us to understand the difference between climate, and weather. To answer it, let’s ask two more questions:
a. Will it be warmer in Madrid in July 2030 than in January 2030?
b. Will it rain in Madrid on 20 June 2020?
I think anyone would answer “yes” to question (a), and “no idea” to question (b). How can we achieve certainty in question (a) while it is impossible with the other?
When we think of (a) we think about what should happen in July and January. And what should happen is related to the height of the Sun in the sky. The higher, the more heat comes the longer, and the more heat it makes. But when we think about question (b) we think about what will really happen on that day, and here we find it impossible to answer more than a week or two in advance.
Question (a) has to do with the climate; (b) with the weather. As the English climatologist Mike Hulme wrote, “Climate is what you expect, weather is what you get” 2. It is possible to predict the climate. For this reason, tourist guides can inform about the average temperature in Madrid throughout the year, without the need for a crystal ball.