Tackling Global Emissions and Limiting Global Warming to 2°C?

Source: dAIR (2014)

It is important to distinguish between CO₂ emissions pathways and cumulative CO₂ emissions (Allen et al, 2009). Cumulative CO2 emissions are total emissions over an extended period of time and these are more significant than a particular emissions pathway (i.e. where emissions peak and the rate at which they rise or fall) in determining changes to the global climate (McGlashan et al, 2012).

Even though climate change mitigation is guided by future temperature projections, the eventual equilibrium global mean temperature associated with a given stabilization level of atmospheric GHG concentrations remains uncertain (Allen et al, 2009). Using ensemble simulations of simple climate-carbon-cycle models constrained by more comprehensive models, Allen et al (2009) simulated the temperature response to various CO₂ emission pathways. Their key finding is that peak warming caused by a given cumulative CO₂ emission is better constrained than warming response to stabilization scenarios. 

Cumulative CO₂ emissions largely determine the global mean surface warming by the late 21st century and therefore, Allen et al (2009) suggested that limiting cumulative CO₂ emissions is key to limiting the scale of anthropogenic climate change. Thus, it is important to situate CO₂ emissions within the framework of the past, present and future (IPCC, 2013). This represents a long-term commitment. 

Some sceptics are unconvinced about our capacity to tackle emissions and also, the feasibility to do so in the current economic and political climate. The basis for their view is that aspects of climate change will persist for hundreds of years even if CO₂ emissions are halted (IPCC, 2013) and it is highly unlikely that global emissions can be completely reduced to zero.

Of course, it is crucial to bear in mind that the carbon cycle has only recently been incorporated into the Coupled Model Intercomparison Project 5 (CMIP 5) models for the IPCC's 5th Assessment Report. In many ways, the inclusion of Earth System Models (ESMs) coupled to the carbon cycle, interactive aerosols and land cover change have improved CMIP 5 model considerations for time-evolving volcanic and solar (radiative) forcing (Taylor et al, 2012). A present problem is the fact that observations currently provide a weak constraint on the carbon cycle and therefore, the carbon cycle's response to future anthropogenic CO₂ emissions (Allen et al, 2009) and we have yet to improve our scientific understanding in this area. 

I agree with the basis for the stabilization sceptic argument, but I feel that it serves humanity no good to sit naively on the side of the fence, thinking pessimistically about our options and not actually doing anything about it. I am very much a pragmatist. My argument is that there are great incentives to invest in and adopt new energy technologies, regardless of the climate change issue. 

Let's take out the climate change issue for a moment. Surely it makes sense that if fossil fuel reserves are declining (BP, 2013), then businesses, governments, scientists and the public should naturally be inclined to explore new options/technologies. 

In relation to emissions, I am very much a supporter of acting imminently even if we are uncertain of the carbon cycle responses to future anthropogenic CO2 emissions. Research and implementation of alternative technologies for energizing the future should focus specifically on the need to limit cumulative CO₂ emissions (McGlashan et al, 2012).

Why do we need to limit cumulative emissions of CO₂? A certain fraction of the carbon dioxide we release accumulates in the atmosphere, in effect, indefinitely because of the way it interacts with ocean chemistry. So it is the total amount that we emit that matters in the long term, not the rate we emit it in any given year or the average rate of emission over a limited period. Both peak temperatures and peak carbon dioxide concentrations, which matter for impacts like ocean acidification, are primarily determined by cumulative carbon dioxide emissions (TrillionthTonne.org, n.d.). 



Why limit warming to less than 2°C?A sustained 2 °C warming would already be well outside the range of temperature fluctuations experienced since the last ice age, and many vulnerable ecosystems and societies are expected to suffer under this level of climate change. Many are calling for global warming to be limited to 1.5 °C if possible. But there is a large body of evidence that the risk of dangerous climate change rises if we fail to limit warming to 2 °C above pre-industrial temperatures, and many of the world's governments have now adopted this limit as their goal. The need to limit cumulative emissions of carbon dioxide applies even more forcibly if we are to limit warming to a lower temperature (TrillionthTonne.org, n.d.).

Source: Roberts (2012) 

An alternative to the 2°C target....or not?

The debate over whether the 2°C global warming above pre-industrial levels is a meaningful target in the climate change policy context has picked up in recent weeks. The 2°C global warming goal has been a critical value for the climate change policy realm, but ahead of the climate summit in Paris 2015, many such as Victor and Kennel (2014) have questioned the target altogether. 

The recent paper by Victor and Kennel (2014) published in October 2014, called for climate policy to ditch the 2°C warming goal. Instead of taking the average global temperature, they proposed that we should look for vital signs such as ocean heat content, climate extremes etc. to better understand climate drivers and risks. They also argued for setting scientifically meaningful goals. Again this goes back to the concept of cumulative emissions. The amount by which accumulating greenhouse gases in the atmosphere are perturbing the planet's energy balance is accelerating (Victor and Kennel, 2014). 

Illustration by David Parkins. Source: Victor and Fennel (2014)

Ok - Fair enough. But I, like many others such as Stefan Rahmstorf, do not agree with Victor and Kennel's paper. Firstly, Victor and Kennel do not even suggest a viable alternative to the 2°C target. A volatility index that measures the evolving risk from extreme events is simply too complex to be translated into a global goal. There are uncertainties relating to the drivers of extreme events and whether or not they are forced by humans (Rahmstorf, 2014). The 2°C target is a simple, clear goal that not only conveys the urgency of the need for action, but also allows policy-makers to make decisions that have global significance. 

To summarise, the debate over climate change targets and the best metrics for measuring anthropogenic impacts will inevitably carry on for some time. What we can conclude from this is the need to reduce cumulative emissions and one of the ways this can be achieved is through technology. Obviously, technology cannot solve all problems and significant effort must be put in place to make technologies cost competitve and scale-up successes. Regardless of the climate change issue, there are great incentives to invest in and adopt new energy technologies to move beyond fossil fuel dependency.  



References

Allen, M. R., Frame, D. J., Huntingford, C., Jones, C. D., Lowe, J. A., Meinshausen, M., & Meinshausen, N. (2009). Warming caused by cumulative carbon emissions towards the trillionth tonne. Nature, 458(7242), 1163-1166.

McGlashan, N. R., Workman, M. H. W., Caldecott, B., Shah, N. (2012). Negative Emission Technologies. Grantham Institute for Climate Change Briefing Paper No. 8. Imperial College London. 

Stocker, T. F., Qin, D., Plattner, G. K., Tignor, M., Allen, S. K., Boschung, J., ... & Midgley, P. M. (2013). Climate change 2013: The physical science basis.Intergovernmental Panel on Climate Change, Working Group I Contribution to the IPCC Fifth Assessment Report (AR5)(Cambridge Univ Press, New York).

Taylor, K. E., Stouffer, R. J., & Meehl, G. A. (2012). An overview of CMIP5 and the experiment design. Bulletin of the American Meteorological Society, 93(4), 485-498.

Victor, D. G., & Kennel, C. F. (2014). Climate policy: Ditch the 2° C warming goal. Nature, 514(7520), 30.