Global energy sector emissions decreased slightly in 2015, but CO2 in the atmosphere spiked ... What gives?

Two reports released this week about global energy sector carbon emissions and atmospheric CO2 concentration may be confusing. At least, they were to me initially.

On Wednesday the International Energy Agency reported that carbon emissions from the global energy sector remained roughly flat for the second year in a row. According to IEA, 2014 emissions were 32.3 billion metric tons and 2015 emissions were 32.1 billion metric tons. In fact, then, emissions from the global energy sector decreased slightly year-over-year. Those same years, global GDP increased 3.4% and 3.1%, respectively. (See, e.g., Chris Mooney’s March 16 Washington Post piece). This is significant because it indicates that since 2013 the global economy grew while economy-related carbon emissions did not. It is welcome news, obviously, confirming that we are seeing a decoupling of greenhouse gas emissions from economic activity. Put another way, energy intensity – the quantity of energy required to produce a unit of economic output – is decreasing. We're getting more efficient – Yay! – as we must if we are to succeed in mitigating global warming's worst-case outcomes caused by GHG emissions from human activities, such as combusting fossil fuels to power our lives.

Also this week the U.S. National Oceanic and Atmospheric Administration (NOAA) reported that the atmospheric concentration of CO2 increased by 3.05 parts per million in 2015, the largest year-over-year spike observed in the 56 years NOAA has been measuring atmospheric CO2 concentration at its Mauna Loa Observatory in Hawaii.

How can carbon emissions from the global energy sector be flattening or slightly declining while the observed concentration of atmospheric CO2 is spiking? As these reports had appeared in my Facebook newsfeed in posts by Penn State climate scientist Dr. Michael Mann and Washington Post Energy and Environment writer Chris Mooney, I reached out to them via Twitter seeking clarification. Professor Mann responded, “Emissions & concentrations don't track on annual basis. The latter is a lagging indicator.” Chris Mooney responded, “One thing to note is that IEA is only energy related emissions. El Nino also created fires, droughts.” Science blogger Greg Laden also replied, pointing out that drying tropical forests grow less, take up less CO2, and are more susceptible to fire.

Confusion resolved. Energy sector emissions may have stalled, but the lag between emissions and observed atmospheric CO2 concentration, along with other factors like El Nino, drying rain forests and drought, and other sources of emissions like the massive Indonesian forest fires mean that 2015 still saw a record spike in atmospheric CO2 concentration.

As Professor Mann’s reply highlights, it takes time for CO2 emitted as a result of human activities to mix in the atmosphere. The CO2 concentration increases we measure for a given year reflect emissions and other contributing factors from years past. As he notes, concentration lags emissions. On the matter of lags, I would add that because of inertial lags in the climate system it takes decades – about 40 years –for GHGs emitted in a given year to affect global average surface temperature.(*) The scary implication of the 40-year lag is that the global average surface temperatures we are experiencing now – almost 1°C above the mid-20th century average – are the product of emissions accumulated up through … the 1970s. And we’ve emitted tens of billions of tons of CO2 and other GHGs over the decades since. CO2 emissions have increased by more than 90% since 1970 according to the Environmental Protection Agency. The temperatures that will prevail around the globe 40 years from now will reflect the CO2 and other GHGs we emit today (and what has accumulated up through today). That is both cause for worry and reason to act aggressively … now! … to bring down carbon emissions.

Which brings me to this important take-away. The significance of the IEA data cannot be overstated. The fact that global economic production grew while carbon emissions simultaneously declined proves that taking action to mitigate global warming by switching from fossil fuel-based energy generation to renewable sources and energy efficiency does not correspond to a decline in economic growth. These recent IEA data together with a growing body of evidence show the opposite, in fact. In recent years, numerous independent analyses focused at levels of geographic scope from municipalities to states to nations to global have found that moving to cleaner, renewable energy sources, improving energy efficiency, and sustainably managing natural resources produce economic benefits that 1) are greater than business-as-usual, do-nothing scenarios and 2) exceed their costs. Often, the analyses reach this conclusion even without factoring in adverse climate impacts avoided and public health benefits gained. Not that their arguments were ever strong, but those who object to climate action on economic grounds can no longer do so credibly. Certainly, given the ample evidence available, their tired old shibboleths should not be allowed to go unrebutted. Rather, in fact, advocates need to marshal the evidence – economic, public health, ecological, security, etc. – and forcefully, articulately make the case for concerted climate action now.

(*) This should not be confused with the lag between temperature and CO2 coming out of glaciation periods as understood from ice core and other data. For more on that, see, e.g., here, here, and here.