Do we need to intervene?

An often-heard argument from sceptics of anthropogenic climate change is that both the climate and atmospheric concentrations of carbon vary through the history of the Earth, and so who’s to say that the temperature changes that are occurring now are not a part of a natural climate cycle. It was even suggested to me, from an unnamed friend, that it is “incredibly ignorant” of humans to believe that they could influence a system as significant as our planet’s climate.

The Earth has seen vastly different climatic conditions through its 4.6 billion-year history, and on a range of timescales: tectonically-driven, 10⁵ to 10⁷ year processes; 10⁴ to 10⁶ year orbital cycles; and least commonly, abrupt changes that last anything from 1,000 years to 100,000 years (Zachos et al., 2001). These variations have involved differing temperatures and carbon dioxide concentrations, and if you go back around three million years, you’d be able to measure CO₂ levels as high as those found today.

So, if the climate is naturally variable, what evidence is there that humans have even contributed?

Signs of a human effect on atmospheric CO₂ exist from as far back as 7,000 years ago (Ruddiman, 2013). The latest interglacial, the Holocene, began around 12ka BP. At 7ka BP, a shift occurred which set the period apart from historical averages: the carbon dioxide and methane concentrations began to rise, contrasting the three previous interglacials in which CO₂ and CH₄ in the atmosphere gradually declined. Ruddiman shows this event matches with the onset of forest clearing and agricultural advances; such a seismic shift in the Earth’s behavioural pattern during similar interglacial periods as today must plant climate change concerns into the heartiest of sceptics.

This evidence, however, only scratches the surface of the anthropogenic influence on the atmosphere. Carbon dioxide concentrations - that had remained mostly constant not only through the last 1,000 years (Figure 1), but also through interglacial periods during the last 250,000 years - increased sharply following the Industrial Revolution (Figure 2). Fossil fuel energy production brought with it unfathomable population increases (current levels being seven-times the one billion alive in 1820) and a global economy 50-times greater than in 1800 (Steffen et al., 2007), both increasing the demand for energy even more: a domino effect on a planetary scale.

Figure 1 - CO2 through the last 1,000 years. (Ruddiman, 2013)

Figure 2 - CO2 concentrations during interglacial periods over the last 250,000 years. (Steffen et al., 2007) 

And, perhaps most concerning of all, is the unprecedented rate at which this increase in CO₂ has occurred: 75% of the CO₂ rise attributed to anthropogenic actions has been released since 1950 (Steffen et al., 2007), and this is predicted to have contributed 0.5-1.3^oC to the average global surface temperature (IPCC, 2014). In the ~250 years since 1750, the CO₂ concentrations measured in the atmosphere have sharply increased from a relatively-constant interglacial marker to levels not present on earth at any point in the last 800,000 years (Figure 3).

CO2 concentrations over the last 800,000 years (Source)

But what does this mean for the environment? Why do we need to cut our emissions?

As stated above, just the emissions since 1950 have caused an estimated 0.5-1.3^oC rise in global temperature. Steffen writes that there are three general scenarios that humankind could adopt in the future in regards to climate change:

(1)    Business-as-usual: Emissions continue to rise throughout the 21^st century.

(2)    Mitigation: A direct approach to tackle the problem, with the most extreme target being the return of the earth system to levels present before major human impact.

(3)    Geoengineering: A drastic form of mitigation, involving the intentional alteration of an earth system. An example is the pumping of aerosols into the atmosphere to reduce the amount of solar insolation reaching the earth surface.

The IPCC built upon these ideas in the Fifth Assessment Report (2014) with four Representative Concentration Pathways (RCPs), each presenting a different emission scenario based on the point at which anthropogenic release of CO₂ reaches its peak. Under RCP8.5, the ‘business as usual’ scenario in which emissions continue to increase throughout the 21^st century, global mean surface temperature is predicted to rise by a mean value of 3.7^oC, with the maximum of the forecasted range being as high as 4.8^oC (IPCC,2014). An inflation of temperatures such as this would have catastrophic consequences, most notably threats to human health, species survival and sea level rise (IPCC, 2014). A fantastic run-through of the different emission scenarios and their potential impacts is provided by Skeptical Science.