By James Renwick

The news about climate change has sounded quite dire for many years, with looming threats around extreme rainfalls, heat waves, fires and other extreme events. If the world does not take action on reducing emissions of greenhouse gases, especially carbon dioxide, the future could be very grim indeed.

The counter to this “gloom and doom” scenario is the knowledge that the problem is totally under our control. The climate is changing because of human emissions of greenhouse gases, gases that accumulate in the atmosphere, absorbing heat and warming the earth’s surface. Human activity is making the problem happen, and human activity can stop it from happening. We have the power, we just need to use it.

If the global community can reduce carbon dioxide emissions to zero by 2050, and get methane and other emissions under control, we stand a good chance of stopping global warming at 1.5 degrees Celsius, a level that would probably not be too overwhelming for global society. That was the conclusion reached in a landmark report published by the Intergovernmental Panel on Climate Change (IPCC) in 2018. At the time, the challenge was to get global emissions down by half by 2030, and then to zero by 2050. New Zealand’s Zero Carbon Act is largely based on the science in that IPCC report.

Achieving zero carbon dioxide by 2050 (and half by 2030) is a daunting task, but one that is possible, given enough political will and buy-in from the business sector and society at large.

The problem is that in just the last couple of years, there have been some science advances that make the time frames for action look even tighter. First, new analysis of the historical temperature record suggests that global temperatures have risen a little more than previously thought. Global temperatures are already 1.1°C above the pre-industrial base line, so 1.5 degrees of warming is not far away. Unless global emissions of greenhouse gases drop dramatically in the next few years (much more than the temporary drop brought about by the Covid-19 pandemic), 1.5°C warming will be here by 2030. To have any hope of stopping global warming at that level, concerted action has to start this year.

The second issue is that we may have been underestimating how quickly the climate warms as carbon dioxide is added to the atmosphere. The way climate scientists measure this is through the “climate sensitivity”, an estimate of how much global average temperatures increase when atmospheric carbon dioxide levels are doubled. There are several ways to estimate this. We can look to the past and study how much global temperatures rose and fell through the ice ages, and relate that to greenhouse gas changes and other factors. We can look at how temperatures have risen over the last century of observational records and relate that to greenhouse gas increases during the same period.

We can also use climate models, computer simulations of the physics of the climate system. In a model, the amount of carbon dioxide in the model atmosphere can be doubled, then the model run until the climate adjusts and comes to an equilibrium. The difference in global average temperature between the start and the end of the simulation gives us the model value for what’s known as the equilibrium climate sensitivity.

The latest generation of climate models, with higher spatial resolution and more sophisticated treatment of clouds and other elements of the climate, are showing higher sensitivities than prior versions of those models, a result that has surprised and concerned the climate research community. Their projections are being analysed now and will be summarised in the next major IPCC report, the 6th Assessment Report, due out in mid-2021.

For over 40 years, equilibrium climate sensitivity was estimated to be around 3°C, with a likely range of 1.5° to 4.5°C, based on geological history, recent observations, and climate model results. The latest round of model results suggests the climate sensitivity could be 5°C or more. If those results were correct, they would mean we could expect temperatures to rise faster than we’ve thought in the past. Hence, the globe may reach 1.5° or 2°C warming more quickly than previously thought, meaning that we have even less time to take the action needed.

There is plenty of research yet to do to understand what causes the higher model sensitivities, and whether or not they are consistent with the other lines of evidence we have, from the history of the climate. At this stage, we cannot rule out the possibility that climate sensitivity is higher than we’ve been banking on for many years.

What does this all mean for us in New Zealand? If the climate changes faster than previously thought, we will see extreme events such as heat waves, floods, and droughts intensifying faster, putting native ecosystems and our food and water security in greater danger, faster. It would mean more rapid sea level rise and the potential for much more rapid melting of ice from the big ice sheets in Greenland and Antarctica, potentially locking in metres of sea level rise in coming centuries. It would mean more disruption across the globe.

It would also mean that we need to work harder to rein in the changing climate. Instead of getting to zero emissions by 2050 to stop warming at 1.5°C, perhaps we need to get there by 2045 or even 2040. The power is still in our hands though. We are the ones changing the climate and we are the ones who can stop it changing. The faster we can decarbonise our economies and our lifestyles, the sooner the warming will stop. It’s that simple.


James Renwick is a Professor of Physical Geography at Victoria University. He is an expert in climate change.

See Also:

How will climate change impact Aotearoa? ▶

Climate change: Should we be worried?