Increasing Greenhouse Gases

Overview

One thing about the global warming debate that we know for sure is that concentrations of several greenhouse gases in the atmosphere are increasing due to the activities of man. The table below lists the greenhouse gases that are increasing due to human activity as well as the percentage increases since 1750. The last column indicates the major natural and anthopogenic sources for each of the gases. Note that some of these greenhouse gases produced by humans have no natural sources. As mentioned previously, the production and use of ozone-depleting CFCs has been greatly reduced in recent years, and the concentrations of these chemicals in the atmosphere have started to decline. However, the the ozone-friendly chemical substitutes that are now used for air conditioning, HFCs are also greenhouse gases. Human activities release other greenhouse gases, such as PFCs, and SF₆. Another greenhouse gas, ozone, has recently gotten some attention as a potential contributor to global warming and climate change. The concern is over increasing concentrations of ozone in the troposphere related to photochemical smog. In the table below, greenhouse gas concentrations are given in either ppm (parts per million), ppb (parts per billion), or ppt (parts per trillion), where, for example, one ppm means one molecule of greenhouse gas per one million gas molecules in the atmosphere. The figure below the table on the left shows that carbon dioxide makes up 76% of all human emissions of greenhouse gases, with methane and nitrous oxide making up another 22%. The F-gases, which include the other gases in the table, except ozone, only make up about 2% of the total greenhouse gas emissions. The figure on the right shows the percentage of greenhouse gas emissions that are associated with different economic sectors.

Percentage of total greenhouse gas emissions by individual gases. Source (IPCC, 2014)	Percetage of total greenhouse gas emissions by economic sector. Source (IPCC, 2014)

Six of the greenhouse gases listed in the table above, carbon dioxide, methane, nitrous oxide, HFCs, PFCs, and sulphur hexafluoride, were targeted for reduction by the Kyoto Protocol of 1997. Countries that signed the Protocol agreed to reduce their cumulative emissions of these gases to 5% below 1990 levels over the period from 2008 to 2012. The reductions are based on CO₂ equivalents (see global warming potential table below). The Protocol did allow flexibility to countries to apply carbon offseting and market-based mechanisms, such as carbon trading, to meet their overall emission reduction goals. Some of the countries, which signed the Protocol, did meet their pledge to limit emissions, while others actually continued to increase their emissions. The successful and failing countries are shown in Has the Kyoto protocol made any difference to carbon emissions? The article concludes that there were more successes than failures and that total emissions from countries that signed the Kyoto Protocol decreased from 1990 to 2012. However, overall global emissions have significantly increased over that period, especially due to increases from China and other emerging economies. One should also consider the fact that some of the emissions from places like China were produced while manufacturing goods that were shipped to economies that signed the Protocol. Note that the United States did not sign or agree to the emission reductions set by the Kyoto Protocol.

While most agree that the modest emission reductions agreed to in the Kyoto Protocol will have only a very slight impact on the rate of global warming, some say it was an important first step in getting the global community to recognize the issue and work together on possible solutions. The Kyoto agreement expired in 2012. World climate conferences held in Copenhagen, Denmark in 2009, Cancun, Mexico in 2010, and Durbin, South Africa in 2011 did not lead to any significant agreements for futher reductions in greenhouse gas emissions. An important on-going issue is that the largest increases in greenhouse gas emissions are coming from developing countries like China and India. In fact China has now surpassed the U.S. as the #1 emitter of greenhouse gases (this is for the entire country, the U.S. still emits the most per person). Some developing countries have shown little interest in agreeing to emission reductions, arguing that they are only trying to achieve the standard of living enjoyed in the developed world. If China will not agree to emission reductions, then there will be those in the U.S. who will argue that we should not agree reductions either, mainly because emission controls will lead to higher energy prices and put the signing countries at an economic disadvantage. The other big reason holding up an international agreement is uncertainty about the ultimate effects of higher greenhouse gas levels on climate change.

In December 2012, the Doha Amendment to the Kyoto Protocol was adopted. Most of the countries that agreed to the first set of emission reductions by 2012 have now comitted to reduce their carbon equivalent emissions of greenhouse gases to levels at least 18% below 1990 levels during the period from 2013 to 2020. Once again the US did not agree to the Protocol. Two important countries that were part of the original Kyoto Protocol, Canada and Japan, decided to opt out of the Kyoto Protocol at the end of 2012 and did not agree to the new emission reductions. The new agreement includes one additional greenhouse gas, nitrogen trifluouride (NF₃).

In December 2015, at the Paris Climate Summit, 195 countries, which covers over 90% of the global emissions of greenhouse gases, voluntarily pledged to reduce greenhouse gas emissions during the decade of the 2020s. This covers the period after the Doha Amendment to the Kyoto Protocol. The stated overall goal is to limit the increase in global average surface temperature to less than 2°C above the pre-industrial average temperature and to less than 1.5°C if possible. (Instructor's Note. There is no certainty that this goal would be met if all countries meet their pledged greenhouse gas emission reductions. In fact the connection between temperature changes and greenhouse gas emissions is highly uncertain as there is much that we do not understand about climate and climate change. A better overall goal would be to set a limit on the future amount of greenhouse gases in the atmosphere as we have more direct control over that). A summary of each of the countries' pledges to reduce emissions is published in Paris 2015: Tracking country climate pledges. The reaction to Paris Agreement is mixed. It is praised in the New York Times, see Nations Approve Landmark Climate Accord in Paris. However, others are worried that the agreement consists mostly of promises and goals, but no firm or legally-binding commitments, e.g., James Hansen, father of climate change awareness, calls Paris talks 'a fraud' published in The Guardian.

Changes in the Rate of Increase of Carbon Dioxide

One potentially alarming aspect about the observed rate of increase of CO₂ since 1900 is that the rate of increase itself has continued to increase. Mathematically, this type of growth rate is better described as growing exponential rather than linear. For a linear increase the amount of CO₂ in the atmosphere would increase by the same amount each year, for example 2 ppm per year. For linear growth the year to year percentage increases relative to the total amount of CO₂ in the atmosphere will get smaller as the total increases. But for an exponential increase, the percentage increase from year to year remains the same, which means the year to year increase gets larger each year. Since 1900, not only has there been year to year increases in the amount of CO₂ in the atmosphere, but the overall percentage increase from year to year has also been increasing. This is an exponential function with an increasing growth rate. Do not worry if you do not understand the mathematical terms. The point can be made using data of measured changes in CO₂ in the atmosphere since 1900. The table below shows that the rate of increase of carbon dioxide has itself increased over each 25 year period of the last century. In 1925, CO₂ was 2.9% more than 1900; In 1950, CO₂ was 3.9% more than it was in 1925; In 1975, CO₂ was 5.7% more than 1950; In 2000, CO₂ was 8.9% more than 1975. Also note that there has been an additional 10.4% increase in carbon dioxide over the 16 year period from 2001-2016. Thus, it is easy to conclude based on these observations that little if any progress had been made in reducing greenhouse gas emissions worldwide through the twentieth century. In fact it was more correct to conclude that the rate at which we were adding greenhouse gases to the atmosphere had accelerated over most of the time period shown in the table.

Year	CO2 concentration (ppm)	% increase in 25 years
1900	297	---
1925	306	2.9%
1950	318	3.9%
1975	336	5.7%
2000	366	8.9%
2016	404	10.4% (16 yrs)

More up-to-date measurements of annual average carbon dioxide concentrations since 1959 are shown below. The annual average concentration of carbon dioxide was measured at the Mauna Loa Observatory on the island of Hawaii. Shown on the right are the year to year increases in carbon dioxide. The horizontal black lines show the average increase for each decade, i.e., 1961-1970, 1971-1980, 1981-1990, 1991-2000, and 2001-2010. The average rate of annual increase in CO₂ grew significantly from the 1960s through the 1970s and 1980s. The 1990s had a slight decrease compared to the 1980s, but then increased again to an annual average increase of nearly 2 ppm per year for the period 2001 to 2010. It appears that the annual average for the current decade 2011 to 2020 will be even greater than 2 ppm per year. The last two years, 2015 and 2016, had the largest annual increases in CO₂ observed since 1959 of over 3 ppm. One reason for the record-breaking yearly increase is that 2015 and 2016 were strong El Nino year. Note that the next largest yearly increase happened in 1998, which was the last time there was an El Nino event of this magnitude. Notice that in 1999, which is the year following the large El Nino, there was a sharp drop in annual growth. We will have to wait and see if the same things happens following this most recent El Nino.

Although CO₂ concentrations continue to increase in Earth's atmosphere, there is some potentially good news on this issue. The emissions of greenhouse gases from most developed countries has decreased following the global economic slowdown in 2007, including the United States where emissions have fallen by 11%. The reduction in US emissions of greenhouse gases is also shown in this plot of yearly total US greenhouse gas emissions 1990 to 2012. Some of this is undoubtedly due to the slower economy. However, other reasons include the largescale shift to natural gas power generation, more fuel-efficient vehicles, and the development of renewable energy. Please read over the following short article, Carbon dioxide emissions from electricity generation in 2015 were lowest since 1993. At the moment, the reduction of emissions from developed countries is being offset by increased emissions from developing countries and the year to year increase in CO₂ has somewhat stabalized (although there are still year to year fluctuations depending on factors such as El Nino and La Nina). Here is a link to an article that projects future increases in energy-related carbon emissions will come mostly from developing countries, Projected growth in CO2 emissions driven by countries outside the OECD.

Average yearly concentration of CO2 from 1959 to 2016 as observed from Mauna Loa, Hawaii.	Increase in CO2 relative to previous year from 1960 to 2016

Global Warming Potential

On a per molecule basis, not all greenhouse gases are equal in their enhancement to the greenhouse effect. The table below defines something called the global warming potential (GWP). The GWP estimates the relative enhancement to the atmospheric greenhouse effect on a per molecule basis. Carbon dioxide is assigned a GWP of 1 so that all other greenhouse gases emitted by humans can be compared with carbon dioxide. For example, one molecule of nitrous oxide is about 310 times more effective than carbon dioxide in its enhancement to the greenhouse effect on Earth. Some CFCs and their substitutes are up to 10,000 times more effective. According to the table below, one molecule of sulfur hexaflouride is 23,900 times more effective than one molecule of carbon dioxide in enhancing the greenhouse effect on Earth.

Be careful not to be mislead by the table. The total enhancement to the greenhouse effect depends on the per molecule global warming potential times the number of molecules added to the atmosphere. Overall, carbon dioxide is still the largest contributor to global warming (enhanced greenhouse effect) because there is so much more of it emitted. A figure presented above shows that of all the greenhouse gases emitted worldwide, about 76% is CO₂, 16% is CH₄, 6% is N₂O, and the remaining 2% includes all of the HFCs, PFCs, and sulfur hexafluoride.

Presently, it is believed that the enhancement caused by carbon dioxide is still greater than the enhancements caused by all other human added greenhouse gases combined. Researchers have computed a number called "radiative forcing" that is meant to account for the influence that human-added greenhouse gases have had in altering the incoming and outgoing radiation energy for the planet Earth. Please see this figure showing radiative forcing for many human-added greenhouse gases, which shows the calculated radiative forcing for several greenhouse gases from 1979 to 2010.

Perspective for Increasing Greenhouse Gases

There is no arguing that the concentrations of several greenhouse gases are increasing due to the activities of man. We can measure the increases. This increase in greenhouse gases is a perturbation or forcing to the climate system. Based on our discussion of the energy budget for the Earth, adding greenhouse gases should act to warm the surface of the Earth by enhancing the natural greenhouse effect. However, the way in which the climate system will ultimately respond to this perturbation of adding greenhouse gases is uncertain because the climate system is so complex. We simply do not understand all of the complex interactions and feedbacks that operate in the climate system. One source of uncertainty that we will discuss next is called the "Missing CO₂ Sink". Even though we know how much CO₂ is being released into the atmosphere, we do not know where it all goes. How can we expect to accurately predict future climate changes when we don't even know what happens to all of the CO₂ that we add to the atmosphere? This is just one of many sources of uncertainty related to predictions of future climate changes.