As science was advancing in the 19th century, researchers began to understand the idea that there could be natural changes in the climate over time.
Some scientists began to propose that humans could have influence over these changes.
But it wasn’t until the 1960s that evidence of a warming effect from carbon dioxide became increasingly convincing.
We know there are many things that can warm (or cool) the planet— the circulation patterns of the ocean, radiation from the sun, plate tectonics and volcanic eruptions.
But how do we know that humans have driven the most recent warming?
There’s something happening here…
Yes, something’s happening.
And we have the ability to look back and see temperature records and geological evidence that tells us that something is happening. The records also tell us that recent trends sit outside the range of natural variability.
We also know something’s happening because we have been observing the effects in lots of different places.
For example, ice sheets and glaciers are shrinking, while sea levels are rising. Snow melts sooner in the spring, plants flower earlier. Extreme weather has become more extreme.
Since the 1970s, scientists have tried to model what future trends might look like based on greenhouse gas emissions and temperature rise.
A number of these models are based on the idea of climate sensitivity—a measure of how warming responds to greenhouse gases.
If humans are to have an influence of the climate, there needs to be an observable relation between emissions, temperature and other effects.
One of the earliest modellers was Professor Wally Broecker, who in 1975 estimated that by the year 2010, CO2 would be around 403 parts per million (ppm) and temperature increase around 1.1°C above pre-1900 levels, an estimate that was remarkably close to observed levels of 393 ppm and 0.9°C of warming.
Since then, modelling for climate has advanced at scale and continues to project climatic outcomes with reasonable accuracy.
Ruling out other explanations
Let’s begin with a graphic from an earlier post, which showed how unusual recent warming is when compared with the last 2,000 years.
Now let’s isolate temperature change since 1880.
One of the more common myths about causes of climate change is solar activity. The theory being that increased solar activity has caused the globe to get warmer.
The Sun explains some of the increase in average global temperature, but its a relatively small amount and has had little effect on the overall climate.
Could it be volcanoes?
Not really. Humans emit over 100 times more CO2 than volcanoes and volcanic eruptions also emit short-lived cooling sulfates.
Is it orbital processes?
As mentioned, orbital processes affect the climate over longer time scales—in the tens of thousands of years.
In fact, almost every factor is inconsistent with the level or warming observed in recent decades. You can look at each and every one of them in detail, with peer-reviewed evidence at Skeptical Science.
The final chart is greenhouse gases, which are almost 50 per cent higher than they were in the pre-industrial era.
It’s no contest.
Update February 2022: See also Climate Change Deniers vs The Consensus