For that other endangered species, the climate change denier. A primer for the rest of us.

Despite surveys showing that a majority of the world’s population agreeing that climate change is a global emergency, recent reports that the head of the World Bank is to step down from his role a year early following comments he made last September on climate change, of outright denialism being shared online following Cyclone Gabrielle in New Zealand, and far right attacks on net zero policies show that a vocal minority remains opposed to climate change action.

Here's something I’ve dusted off and updated from a couple of years ago setting out why there should be no doubt climate change is caused by us, and one of the most pressing issues facing humanity.

Firstly, have a look at this illustration from NASA that explains the greenhouse effect:

The greenhouse effect is a natural phenomenon and is essential for life to exist:

“Greenhouse gases in the atmosphere, including water vapour, carbon dioxide, methane, and nitrous oxide, act to make the surface much warmer than this, because they absorb and emit heat energy in all directions (including downwards), keeping Earth’s surface and lower atmosphere warm. Without this greenhouse effect, life as we know it could not have evolved on our planet.”

It’s also true that there are variations in our climate:

“Geological records stretching back millions of years indicate a number of large variations in Earth’s climate. These have been caused by many natural factors, including changes in the sun, volcanoes, Earth’s orbit and CO2 levels.”

We also know that when humans burn fossil fuels, for example oil, natural gas and coal, carbon is released into the atmosphere. That carbon combines with oxygen to make carbon dioxide and increases the greenhouse effect, warming our atmosphere.

Since the Industrial Revolution the amount of fossil fuels consumed has increased exponentially, meaning the amount of carbon dioxide in the atmosphere has also increased.

We know that temperatures are currently rising - the International Panel on Climate Change’s Sixth Assessment Report Climate Change 2021: The Physical Science Basis produced the graph below and also noted “widespread and rapid changes in the atmosphere, ocean, cryosphere and biosphere have occurred” and “the scale of recent changes across the climate system as a whole – and the present state of many aspects of the climate system – are unprecedented over many centuries to many thousands of years.”

We also know that the level of carbon dioxide (CO2) in the atmosphere is rising and currently sits at 423 parts per million:

In fact, CO2 levels haven’t been anywhere near this high in over 800,000 years. We know this from analysing ice cores drilled in Antarctica.

CO2 levels were last over 400 ppm three to five million years ago, during the Pliocene. Back then the world was a very different place:

“Antarctica was a plant-covered oasis, sea levels were an estimated 10 to 20 meters higher, and global temperatures were an average of 2 to 3 degrees Celsius warmer. In the Arctic, summer temperatures were a full 14 degrees higher than they are now.”

From our knowledge of the greenhouse effect we know that increased levels of CO2 cause an increase in temperature:

“Adding more greenhouse gases to the atmosphere makes it even more effective at preventing heat from escaping into space. When the energy leaving is less than the energy entering, Earth warms until a new balance is established.”

So how do we know this increase in CO2, and the resulting rise in temperature, is caused by humans and not a natural occurrence?

Let’s leave aside for a moment that it would be hugely coincidental for fossil fuel consumption,  carbon dioxide levels, and temperatures to all increase so far above previous levels independently and in the same period.

We also know because it has led the International Panel on Climate Change (IPCC) to conclude:

“It is unequivocal that human influence has warmed the atmosphere, ocean and land. Observed increases in well-mixed greenhouse gas (GHG) concentrations since around 1750 are unequivocally caused by human activities.”

The report that included the quote above came from the IPCC’s Sixth Assessment Report Climate Change 2021: The Physical Science Basis. Let’s just take a moment to understand what the IPCC is.

The IPCC is the international body for assessing the science related to climate change. It was set up in 1988 to provide policy makers with regular assessments of the scientific basis of climate change, its impacts and future risks, and options for adaptation and mitigation. These assessments are written by hundreds of leading experts (nominated by governments and IPCC observer organisations) who volunteer their time and enlist many other experts as Contributing Authors and Expert Reviewers. On completion of a report IPCC member governments (195 countries, including the United States and China) consider whether to endorse that report. “Endorsement by governments acknowledges that the report is a definitive assessment”. What all that means is that anything published by the IPCC has been through a lengthy process to ensure assertions such as the one quoted are as robust as possible.

That in itself should be enough. To be sure though, let’s rule out other factors that have caused higher temperatures in the past. One common argument put up by climate sceptics is that volcanoes are responsible for the rise in CO2. However the amount of carbon dioxide emitted by volcanic activity (130-230 million tonnes per year) is dwarfed by the amount produced by humans burning fossil fuels (26 billion tonnes or over 100 times more). Alternative, it is argued that increased energy from the Sun is responsible for climate change. The problem here is that satellite observations since the late 1970s have shown a slight decrease in solar output and instead of cooling, the Earth has warmed.

“Also, warming from the sun would heat all of the atmosphere, including the lowest few kilometres (the troposphere) and the layer above (the stratosphere). Observations show that the stratosphere is in fact cooling while the troposphere warms. This is consistent with greenhouse gas heating and not solar heating.”

Finally, if that’s all not enough, have a look at this video by Carbon Brief explaining why human activity is responsible for global warming, rather than natural factors.