Leo's got the right idea

A post today about something that upset me. I was rewatching Leonardo DiCaprio's Oscar acceptance speech in which he talked about the serious threat from climate change, and in the comment section of the video I came across this comment (sadly with over 100 likes) from a global warming denier (apologies in advance for the comment's poor English):

"Explain climate change if no warming for 18 years? Explain the Chilean volcano injected co2 into the atmosphere in 3 days than the entire human race can in 5 years? Explain how co2 causes climate change by getting into the stratosphere when co2 is 1.5 tinea heavier than air, most stones still don't float on water. What percentage of the air is co2? You need to be able to answer all those questions before you have the right to call people that don't believe in climate change are idiots! Leo certainly can't answer them. Just because he's a celebrity doesn't give him the right to make erroneous claims, some some non celebrities might actually believe..."

I just want to answer each of these questions individually. First of all, "there has been no warming for 18 years". Yes, there has been a supposed global warming "hiatus" since 1998 where rates of surface temperature increase have slowed in comparison to model projections, but, to give the conclusion of one of my assignments for U of T concerning the supposed hiatus, overall longer-term trends of surface temperature increases due to anthropogenic greenhouse gas emissions mean the existence or non-existence of a short global warming hiatus is irrelevant. Recent research is also suggesting that there hasn't been a hiatus in the period 1998- until present, rather there simply appeared to be one due to a switching in the methods for recording surface temperatures that were calibrated slightly differently.

Secondly, the Chilean volcano likely being referred to here is the Calbuco volcano which erupted in April 2015. I couldn't find how much CO2 the volcano actually released, but regardless, as shown in the graph below, when the effects of volcanoes (as well as El Nino/La Nina effects and solar variability) are removed from models, the temperature trend from 1979 to 2011 across several different models is that of a steady increase, showing that it is indeed anthropogenic CO2 emissions that are causing the temperature increase, not natural variability.

Source: Foster and Rahmstorf, 2011. 

Thirdly, CO2 is heavier than O2 (not "air" as stated in the denier's comment, as air includes CO2, O2, N2 and a number of other gases and compounds). In the troposphere, there is enough motion in the air from winds (driven by temperature differences around the globe) to keep gases equally mixed together. Mixing between the troposphere and stratosphere mean CO2 can go up into the stratosphere where there are further circulation systems. Mixing takes a few years up to a decade yet CO2 has a lifespan of up to 100,000 years. The problem is that CO2 in the stratosphere tends to cool the stratosphere even though the troposphere and surface warm up. Given that rapid depletion of the ozone layer in the stratosphere begins when temperatures drop below -80°C, an increase in cooling stratospheric CO2 could worsen ozone depletion. Depletion of stratospheric ozone would lead to an increase of ultraviolet radiation reaching the earth's surface which would increase incidence of skin cancer, cataracts, and reduce yields in susceptible crops.

Fourthly, CO2 makes up a tiny percentage of the air, around 0.0314% . Indeed, water vapour is the most important contributor to the natural greenhouse gas effect (which helps keep the global mean temperature around 15°C instead of -18°C as it would be in the absence of an atmosphere). However, the point is that emissions of CO2 have the most important strengthening effect (see the pie chart below) because a) humans are emitting so much (the cumulative human emission of carbon up to 2014 has been around 560 Gt which is the same amount as was in the pre-industrial atmosphere; current CO2 emission rates from human activity are around 11GtC/year), and b) these CO2 molecules can have a lifespan of up to 100,000 years in the atmosphere (meaning each molecule can trap heat for a very long time after it is released).

Source: Danny Harvey, 2016. 

I hope I've "explained global warming" in relation to the above denier's questions and I hope the scientific community continues to try and break through the psychological barriers that stop people from taking action. Credits to Leo for mentioning climate change in his acceptance speech- it is so important that influential celebrities like him bring the issue to the forefront of public thought.

A quick note about my sources: most of the information comes from my study pack put together by Professor Danny Harvey for my Year 3 level course ("Global Warming") at the University of Toronto.