Here are some briefs answers to 6 frequently asked questions on the topic. I also think that in the following order, these answers contribute to explain how we know there’s a climate change going on, and how we know it comes from us.
- How can we talk about the climate in 100 years if we don’t know if it will rain in 20 days?
- How do we know there is a climate change?
- How do we know it’s not the Sun?
- How do we know it comes from an increase of greenhouse gases (GHG)?
- How do we know these GHGs come from human activity?
- Do experts agree?
I will try to answer them as briefly as possible. Let’s go.
How can we talk about the climate in 100 years…?
A very good question indeed. To answer it, let me ask two more questions:
- Will it be warmer in Madrid in July 2030 than in January 2030?
- Will it rain in Madrid on June 20, 2030?
I think anyone would answer “yes” to (1), and “no idea” to (2). How can we achieve certainty for (1) while it’s impossible for (2)?
When we think of (1) we think about what should happen in July and January. And what should happen through one year is mainly related to the height of the Sun in the sky. The higher the hotter. But when we think of (2) we think about what will actually happen on that day, and here we find it impossible to answer more than a week or two in advance.
The graph below shows the daily temperature in Lausanne Switzerland, in 2017. It is obvious that the rather chaotic yellow line runs around a deeper tendency, shown by the blue line.
The blue line is about the climate. The yellow line is about the weather. Likewise, question (1) is about the climate, (2) is about the weather. As Mike Hulme puts it ,
“Climate is what you expect, weather is what you get”
It is perfectly possible to forecast the climate. For this reason, travel guide books can inform about the average temperature in Madrid throughout the year, without the need for a crystal ball.
By contrast, predicting the weather well in advance is impossible. For this reason, unfortunate tourists may get rain when visiting Madrid in June, despite their travel guide book forecasting little of it.
How do we know there is a climate change?
The most well-known indicator is the global temperature (see the featured image above). It is an average of the temperatures over the surface of the planet. Global temperature has been rising for about a century. We have gained about 1°C since the XIX century (this figure of 1°C will play an important role in answering the next questions). However, it should be noted that it is not the only indicator. Let’s look at 6 more, see for example NASA or the UK MetOffice, all pointing to a warm-up.
Let’s start with some downward indicators:
- The extent of Arctic sea ice is shrinking.
- The volume of Greenland ice is shrinking.
- The volume of Antarctica ice is shrinking.
- The volume of land glaciers is shrinking.
Let’s now continue with rising indicators,
- The oceans’ heat content is rising.
- Global temperature is rising.
- Sea level is rising.
We then come to a total of 7 indicators, all pointing to the same direction: a warming. One can find a few more on the MetOffice web for example. Notably, the impact of global warming on fauna and flora is extensively studied, showing even more signs of a change.
Even before we know their origin, these observations lead us to the conclusion that global warming is occurring. Let’s see why.
How do we know it’s not the Sun?
Our planet is getting warmer. Why?
What does our climate depend on? It depends on the amount of solar energy that comes from the Sun, and of the composition of the atmosphere. Similarly, the temperature in my bed depends on the temperature in my room (the Sun), and of the numbers of blankets I have (the atmosphere). If I’m getting warmer under my blanket, it can only be for 2 reasons: it’s warmer in my bedroom or I have an extra blanket.
Could the Sun be responsible for the warming? It is obviously the first suspect. Does it have cycles that could change the climate? Of course. The last ice ages were the fruit of the so-called Milankovitch cycles. Yet, while they prompted ice ages, they cannot be held responsible for the current change. Why? Because the shortest of these cycles is 20,000 years long; far too long to explain a warming which has been going on for about a century.
So if the Milankovitch cycles are too long, are there shorter natural cycles? Yes. The Sun has a 11 years cycle. But it cannot be responsible either for at least 3 reasons:
- It is too short. We’re looking for a warming agent over a time scale of about one century. Here, it’s up, down, up, down, etc. every 11 years. Not an uuuuup lasting 100 years.
- Its upper value doesn’t even show an increase over the last decades, rather the contrary.
- Finally, a very simple calculation shows that this cycle can prompt variations of the global temperature of only 0.1°C . We observe 10 times more (see previous paragraph).
It’s not the Sun.
How do we know it comes from an increase in greenhouse gases?
If it’s not the Sun, then it must be the atmosphere. No choice. It acts like a blanket which thickness depends on its composition. The most effective gases in this respect, the famous “greenhouse gases” are, in order of importance, water vapor (H20), carbon dioxide (CO2), and methane (CH4) . Without them, the average temperature of the planet would be around -15°C .
What about water vapor? It has something special. The amount of water vapor in the atmosphere depends only on the temperature of the atmosphere. What happens if there is too much? It just rains. This has a very interesting consequence: water vapor alone cannot be the cause of the warming. If there is more up there (and there is), that can only be the result of warming, not the cause. Something else must be warming the atmosphere.
What can that “something else” be? Only two suspects remain: carbon dioxide and methane. It has to be one of the two, or both. Let’s check it out. We can begin by plotting the concentration of these gases in the atmosphere over the last 1000 years:
As can be seen, these gases start rising at the beginning of the XIX century, with an acceleration in the XX century. Today, the CO2 concentration has reached 400+ parts per million (ppm), representing an increase of 42% over the pre-industrial era. As for methane, it has more than doubled. We have our heating factors, with the expected time scale. We already knew it from the previous elimination of suspects, and now we’re just checking. Our atmospheric blanket has been thickening for a century or two. Let’s keep checking: what is the increase in temperature that could be expected from such increases in these gases? Another simple calculation gives about 1°C, that is, what is observed .
Current warming comes from an atmospheric GHG increase.
How do we know these GHGs come from human activity?
Also a good question. Where do these CO2 and CH4 rises come from? From the volcanoes? They are often suspected. But we can exculpate them for at least 3 reasons:
- We have data for atmospheric CO2 and CH4 over almost a million years back and they never reached current levels. If volcanoes had been recently doing something they didn’t in a million years, we would notice.
By the way, this point holds for any natural agent supposedly responsible for the current GHG rise: it should be doing something now that it hasn’t been doing in a million years.
- As shown in the figure above, CO2 and CH4 are rising steadily. Volcanoes would have to be very well in synch to cause such a gradual increase.
- Volcanoes emit about 0.3 Giga-tons (Gt) of CO2 per year. We emit 30 Gt, that is, 100 times more.
So where do these GHGs come from? The figure above gives us a clue: the rise began with the industrial revolution, which consisted in burning coal.
Could these GHGs come from human activity? To check this hypothesis, we can consider the amount of GHG emitted by humans every year since 1750, and reconstruct the evolution of the atmospheric concentration in CO2 and CH4, accounting only for human activity. The figure below is the result of this simple calculation for CO2.I computed the blue curve starting with the concentration known in 1750, then adding each year what was emitted . The red curve comes from observations. Both fit remarkably. The difference between red and blue comes from deforestation.
There is no doubt. The rise of GHGs in the atmosphere has its origin in the massive use of fossil fuels. This can be checked in other ways , and all the tests are conclusive. These GHGs are “ours”.
Do climate scientists agree?
There is a climate change. It is due to human activity. And those who contribute to the progress of knowledge in this area, what do they say? What do experts say on the subject? They agree. Several surveys have been conducted in recent years. They concluded that more than 97% of experts agree.
I personally attended the American Geophysical Union Fall Meeting in 2015 and 2018. With more than 20,000 participants, this conference is the largest in the world for this field of knowledge. There, climate scientists present their latest research.
- How many presentations questioned the existence of a climate change? Zero.
- How many presentations questioned the role of human activity as the engine of change? Zero.
This can be checked browsing the scientific program of the last AGU 2018 for example.
It should be noted to conclude that what is happening is not a surprise at all. Almost two centuries ago, in 1824, Joseph Fourier already explained how an atmosphere warms its planet. In 1859, John Tyndall measured how some gases absorb radiation and concluded that without water vapour, the earth would be “held fast in the iron grip of frost”. In 1896, Svante Arrhenius calculated, by hand of course, the temperature increase that would come from a doubling of CO2 in the atmosphere. Applying his formula to the recent increase of 42%, one finds a warming of + 3oC. Not bad for someone who ignored the details of the interaction between the molecules of the atmosphere and the radiation sent from earth to space .
The current warming is not a surprise for the scientific community. It was expected. It is happening (it didn’t stop in 1998), and it comes from us.
 M. Hulme, Why We Disagree About Climate Change (Cambridge University Press, Cambridge, 2009), p. 1.
 See A. Bret, The Energy-Climate Continuum (Springer, 2014), p. 47.
 There are more, but these 3 are the most important.
 See Kendal McGuffie & Ann Henderson-Sellers, A Climate Modelling Primer (Wiley, 2005), ch. 3.
 The calculation can be done using equation (4.9) of my book, and this reference, which allows to evaluate the variation of parameter epsilon under a CO2 increase of 40%.
 About half of the emitted CO2 goes into the oceans (see here, p. 467).
 An example: if the excess CO2 comes from combustion, then the amount of oxygen in the atmosphere should go down. The observations confirm this perfectly (I designed an exam based on this in 2011). More conclusive evidences can be drawn from Carbon 13 or 14 atmospheric concentrations.
 He didn’t know about quantum mechanics.