The most important greenhouse gas, in terms of our influence on climate, is carbon dioxide (CO2).
We measure the concentration of carbon dioxide in the atmosphere in parts per million (ppm) – so 1 ppm would be one part carbon dioxide to one million parts other gases. Or one part carbon dioxide to nine hundred and ninety nine thousand, nine hundred and ninety nine parts other gases, for the pedantic among you.
[graphic to show this, like a glass of squash? We could have a series of maybe 3 images (or a little animated gif) showing a splash of juice mixing in a glass.]
So is carbon dioxide increasing? Yes.
Before 1750, carbon dioxide in the atmosphere was steady at about 280ppm. In fact, it has been about 280ppm for a couple of thousand years. Over the last ten thousand years, it has varied between 260 and 280ppm. If you take the last 600,000 years (which includes at least 5 ice ages), it has varied between about 180 and 280ppm.
Since 1750, it has been rising, and is now at almost 400ppm. It has increased a lot – and is still rising. (See CO2now.org for the concenration today in Hawaii)
When we account for the effect of other greenhouse gases which have also increased, such as methane and nitrous oxide, the effect is equivalent to more than 450ppm of carbon dioixde. When expressing the impact of a range of greenhouse gases in terms of carbon dioide concentrations, scientists use the term ‘parts per million equivalent – ppme’. So today’s levels are more than 450ppme in comparison to values of ~260–280ppm over the last 10,000 years.
The point is that they are increasing a lot. This is not some trivial little tweak. The concentrations are rapidly approaching double the highest previous values since mankind has been around on the planet (about 200,000 years ago); and in fact for a long time before that.