Previously, we discussed what  “climate”, “climate change”, and “climate variability” mean.  In this edition, we will explore what is causing current climate change, and how this change could accelerate in the future.

What is causing current climate change?  

Climate change (long-term change that underlies periodic climate variability such as El Nino) is caused by ‘climate forcings’ – processes that are largely independent of the climate system, but can nonetheless influence it in important ways. Today, we know from climate models and century-old physical understanding that the most important climate forcing today is human emissions of greenhouse gases.  Conversely, we know that dominant present-day climate change trends are not caused by potential climate forcings such as:

• volcanoes, which tend to cool, not warm, the planet
• solar cycle variations or changes to earth’s orbit, which are too weak to cause current changes or
• cosmic ray impacts on clouds. There have been no trends in incoming cosmic ray changes, which have been too weak to impact climate.

How will climate change in the future?  

Knowing that human carbon emissions are the dominant forcings of present-day climate change allows climate modelling groups to make projections of future climate trends, by ‘forcing’ global climate models (GCMs) with future emission scenarios. The range of climate conditions that GCMs produce in response to these scenarios indicates that, even given rapid global shifts to non-carbon energy sources, the climate will still warm significantly relative to the present day and stay at these elevated temperatures for hundreds of years. Conversely, if all available reserves of oil, gas and coal are mined and burned, climate change will take the planet to levels of warming, extreme precipitation, sea level rise, and other changes that are unprecedented for at least tens of millions of years.  This unprecedented change will persist for many tens of thousands of years, simply because once the system is ‘filled up’ with carbon it is very difficult to remove, by natural processes.

Do irreversible and dangerous climate thresholds exist?

Of great concern is the potential for dangerous climate thresholds that, once passed, cause highly damaging impacts.  For example, coastal populations and infrastructure may have already been committed by past carbon emissions, to multiple metres of sea level rise over coming centuries. Dangerous thresholds are also likely to be surpassed within a generation under business as usual emissions, beyond which low-latitude populations are regularly exposed to lethal and persistent heatwaves. Thresholds may also exist, beyond which unprecedented drought risks to water supply in already-dry regions emerge. Ultimately, identifying these impactful thresholds and determining their likelihood is an urgent task that requires a combination of climate science, engineering, environmental and risk management expertise.

Next time, we will discuss how to assess climate change at the local scale for practical climate risk assessment.