Guilty?

Last entry I told you about a discussion with my non-environmental friends. The second argument they made is the following:
The world has seen a lot of climate changes in the last millennium. Why affirm this is our fault?

In its fifth assessment (AR5), the IPCC has agreed that scientists are 95% sure that humans are the "dominant cause" for global warming since 1950.

Let's first follow a simple train of thought:

• Fact #1: In our first entry we discussed the atmospheric CO2 was as high as it has ever been for the last 650.000 years. The atmospheric concentration of other gases like methane or nitrous oxide have also suffered an exponential increase as a result of human activity since the industrial revolution, as we can see in the image below by IPCC. 

Atmospheric concentration of carbon dioxide, methane and nitrous oxide in parts per million/billion over the last 10.000 years, taken from IPCC AR4.

It is worth mentioning that the primary reasons for the increase in CO2 are fossil fuels and land use, while for methane and nitrous oxide it is agriculture. So there is no doubt we are responsible for these increases in emissions.

• Fact #2: CO2, methane and nitrous oxide are Greenhouse Gases (GHG). This means they contribute with the greenhouse effect. If you do not know how the greenhouse effect works, I suggest the following short, very complete and very nice video taken from NASA:

• Fact #3:CO2 is not only a regular GHG. The IPCC has agreed that CO2 is the most threatening GHG due to it's radiative forcing. The IPCC defines in its AR4 the radiative forcing as:

"Radiative forcing is a measure of the influence a factor has in altering the balance of incoming and outgoing energy in the Earth-atmosphere system and is an index of the importance of the factor as a potential climate change mechanism."

The consecuence of the three facts we just discussed is only one: we are, in fact, important contributors to this climate change. 

The IPCC confirmed this. 

Image (a) shows the radiative forcing of different agents. We can see the radiative forcing of CO2 is the highest of them all. But what is most interesting about this image is that it separates anthropogenic factors from natural ones.  

     

We can easily compare the natural radiative forcing (only from solar irradiance) to the total net anthropogenic one to reach one conclusion: anthropogenic factors are the main cause of climate change. The evidence is convincing.

However, at the same image we can see there is a column regarding the Level Of Scientific Understanding (LOSU) indicating the confidence of the information shown. This is why, on it's fourth assessment the IPCC declared they were 90% confident humans were the dominant cause of climate change. However, on their fifth assessment, this percentage increased to 95%.

I leave you with two cartoons today!!

On the next entry we will discuss to what extent is climate change important, and why do we need to fight it right now!!

See you later, alligator! :D