The “hockey stick” was a name given to a graph that depicted the rapid rise in atmospheric CO2 concentrations and global temperature over the past 1000 years. While CO2 and temperature have not remained constant during this time (due to natural variation in the earth’s climate) the dramatic rise since the industrial revolution is clear and abnormal. While the initial graph has some controversy surrounding it, similar graphs show more or less the same pattern. This is not new news.

Comparison of 10 different published reconstructions of mean temperature changes during the last 1000 years

What is in the news is a renewed domestic debate on climate change due to Senators John Kerry (D-Massachussetts) and Lindsay Grahm (R-South Carolina) introduction two weeks ago of a bill into the Senate, confusingly titled “Clean Energy Jobs & American Power Act”. This bill is essentially a pathway to move America towards greenhouse gas reductions of 20% by 2020, and 80% by 2050 (as compared to 2005 emission levels). Efforts at a global climate change mitigation treaty to be undertaken in Copenhagen in December are pushing for similar global targets.

Without getting into specific issues surrounding either of these efforts, I wanted to talk a bit about emissions pathways, as I feel it is a key point that many people do not understand.

Human activity (primarily the combustion of fossil energy sources and land use changes) is causing an additional 7.6 gigatons (1 gigaton = 1 billion metric tons) of carbon to be released into the atmosphere each year.  This is significant when viewed in relation to the natural carbon cycle, where the interaction of the oceans, soils, vegetation, and atmosphere had been in more or less balance for the past 11,000 years, with atmospheric CO2 concentrations within a 20 ppm variation. Human activity has thrown the carbon cycle out of balance, with CO2 concentrations having risen from ~280 ppm to now ~388 ppm, and projected to increase to 450-550 ppm by mid-century.

Lets say, for arguments sake, that the planet is able to reach an 80% reduction in emissions by 2050. Is the climate problem then solved? No. In the year 2050, humanity will still be emitting 20% of what we are emitting today, and atmospheric CO2 concentrations will be still be rising, though much slower. What is required is a net zero economy, where humanity was no longer adding CO2 to the atmosphere. Only then will CO2 emissions no longer be rising and start to decrease.

But what happens after that? What happens at the end of the hockey stick?

A recently published paper titled “How Difficult is it to Recover from Dangerous Levels of Global Warming” the authors modeled how quickly CO2 concentrations and temperatures would decrease, were excess emissions to be halted at a set of points in the future. What their results showed, was that halting emissions generally resulted in temperatures to stop increasing, but it took a long time for temperatures to decrease – hundreds of years. They “demonstrated that only very low rates of temperature reduction follow even massive reductions in emissions” (Lowe et al, 2009).

Modeled CO2 concentrations and global temperature means at hypothetical dates in the future

So what does this mean? The international climate negotiators are trying to limit temperature rise past 2C, as going beyond would be considered “dangerous levels”.

However, should it take longer for the planet to get its act together on curbing global emissions, the 2C threshold could easily be breached leading to strengthened impacts, not to mention potential abrupt, non-linear responses to increased temperature. This would mean that even if we jumped above a “dangerous level”, we could likely not come back down from it for hundreds of years.

The reasons for this are due to the fact that the earth’s carbon cycle is slow to absorb excess carbon. In the short term, decaying oceanic plant and animal life transport organic matter to the ocean floor, while on geologic time scales, rock weathering ultimately will remove excess CO2 from the atmosphere.

The bottom line is that if and when humanity moves to a net-zero emissions scenario, the concentration of CO2 and the associated temperature change are here to stay for the conceivable future. Unless we want to permanently (for all sense and purposes) raise global temperatures, policy efforts should be aimed at herculean efforts to curb emissions as quickly as possible.

Realizing the longevity of CO2 in the atmosphere, what options are available to us if we want to actively ‘draw down’ CO2 levels? I will explore this in a future post, but some of these options include reforestation, promoting greater carbon uptake by soils, and CO2 capture and sequestration from the air. Stay tuned.