Cumulative emissions are the critical factor behind the warming we’re experiencing. A large fraction, about 20 percent, lingers for millennia (over 10,000 years). That means a big chunk of the greenhouse gases emitted at the dawn of the Industrial Revolution is still heating up our planet today. The “long tail” of absorption means that the mean lifetime of the pulse attributable to anthropogenic emissions is around 30,000 to 35,000 years.The countries with the largest cumulative CO2 emissions since 1750, ranking as of the start of 2019:
1) US – 397GtCO2
2) CN – 214Gt
3) fmr USSR – 180
4) DE – 90
5) UK – 77
6) JP – 58
7) IN – 51
8) FR – 37
9) CA – 32
10) PL – 27
That’s true, despite recent gains in energy efficiency and cuts in emissions. These relatively small steps now cannot offset more than a century of reckless emissions that have built up in the atmosphere. Much more drastic steps are now needed to slow climate change. And as the top cumulative emitter, the US bears a greater imperative for curbing its carbon dioxide output and a greater moral responsibility for the impacts of global warming. Yet the United States is now the only country aiming to withdraw from the Paris climate agreement. China now emits more than the US, and India’s emissions are rapidly rising. But these countries have a much smaller share of cumulative global emissions. Their populations are also much bigger than the US and other wealthier countries, so the amount that India and China emit per person is vastly smaller than the United States or the United Kingdom.
Here are some takeaways from this animation:
1) Cumulative emissions are the critical factor behind the warming we’re experiencing
It’s not simply the rate of our output of heat-trapping gases that changes the global climate; the total amount of carbon dioxide emitted is a critical factor as well.
While atmospheric carbon is gradually absorbed by the ocean and plants, a large fraction, about 20 percent, lingers for millennia. That means a big chunk of the greenhouse gases emitted at the dawn of the Industrial Revolution is still heating up our planet today. If we were to magically cease emitting all greenhouse gases at once, the planet would likely continue warming for a period of time. This leads to the next point.
2) The United States has an outsized role in global warming, despite recent progress
When it comes to total greenhouse gas emissions, the United States does a behind-the-back, through-the-legs, backboard-breaking dunk over China and the Soviet Union.
In other words, the largest share of global greenhouse gases emitted since the Industrial Revolution comes from the US. And with great emissions comes great responsibility to mitigate climate change.
And yes, the United States has already made some of the largest cuts to its greenhouse gas emissions of any country in the world. Between 2005 and 2015, US emissions fell 11.5 percent, largely due to switching to less carbon-intensive fuels like natural gas. However, US energy consumption hit a record high last year, and emissions are on the rise again after years of decline.
3) Carbon intensity matters more than population for cumulative emissions
The graph also shows us that the worst greenhouse gas emitters of all time aren’t the most populous countries. Instead, most of the chart toppers are the largest economic powers. You can see the United Kingdom drop down the rankings as its empire disintegrated over the years, losing an economic grip on the world, for example.
That should teach us something about the most populated countries in the world today: India and China.
China and India do contribute a large and growing share of global emissions — which absolutely needs to be slowed down and reversed — but most of the warming we’re seeing now is due to the emissions of wealthier countries like the United States.
And remember the total emissions rate from both China and India has to be divided by more than a billion to yield an apt comparison to countries like the United States.
In 2015, the United States emitted 15.53 metric tons of carbon dioxide per capita. China emitted 6.59 metric tons. India emitted just 1.58 metric tons. As these countries get richer, their per capita emissions are poised to rise further. This is why technology transfer from wealthier countries to less developed economies is shaping up to be a critical component of fighting climate change.
But ultimately the largest share of the burden in cleaning up this mess should fall to those who played the largest role in creating it. This animation leaves no doubt as to the culprits.
Understanding the carbon cycle is a key part of understanding the broader climate change issue. But a number of misconceptions floating around the blogosphere confuse basic concepts to argue that climate change is irrelevant because of the short residence time of carbon molecules in the atmosphere and the large overall carbon stock in the environment.
It turns out that while much of the “pulse” of extra CO2 accumulating in the atmosphere would be absorbed over the next century if emissions miraculously were to end today, about 20 percent of that CO2 would remain for at least tens of thousands of years.
The complex global carbon cycle process involves carbon absorption and release by the atmosphere, oceans, soils, and organic matter, and also emissions from anthropogenic fossil fuel combustion and land-use changes. The figure below shows the best estimate of annual carbon fluxes from main sources and sinks.
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|Figure from Oak Ridge National Laboratories (Units in gigatons of carbon).|
At first glance, it may seem that the narrow black arrows representing anthropogenic sources are relatively insignificant, making up only a few percent of the total carbon released to the atmosphere in any given year. To understand why anthropogenic emissions are of concern, it is important to think of the carbon cycle as a balance of sorts; every year around 230 gigatons of carbon dioxide are released to the atmosphere, and around 230 gigatons of carbon dioxide are absorbed by the world’s oceans and biosphere. This balance forms an equilibrium of sorts, with the level of atmospheric carbon dioxide remaining largely unchanged over time. However, anthropogenic emissions throw this process out of kilter, adding a new source of emissions unmatched by additional sinks.
The carbon dioxide record over the past 10,000 years demonstrates this situation: the modern period exhibits a large spike in atmospheric carbon dioxide coincident to the time humans started burning fossil fuels.
|Atmospheric CO2 concentrations over the past 10,000 years. From the IPCC AR4 WG1 SPM.|
Graphing emissions over the modern period against changes in atmospheric concentrations illustrates a clear relationship between emissions and increasing CO2concentrations.
It is important to note that not all anthropogenic emissions are accumulating in the atmosphere. Indeed, about half of annual CO2 emissions are absorbed by the ocean and vegetation, and this percentage of absorption, called the airborne fraction, is currently the subject of vigorous debate over whether or not it is changing over time. Scientists can model the absorption of anthropogenic carbon by year for different sinks.
|Image from the Global Carbon Project.|
Determining the residence time of carbon dioxide in the atmosphere is a rather complex problem. A common misconception arises from simply looking at the annual carbon flux and the atmospheric stock; after all, with 230 gigatons absorbed by the oceans and land every year, and a total atmospheric stock of 720 gigatons, one might expect the average molecule of CO2 to remain in the atmosphere for only three to four years.
Such an approach poorly frames the issue, however. It is not the residence time of an individual molecule that is relevant. What really matters is just how long it will take for the stock of anthropogenic carbon emissions that has accumulated in the atmosphere to be reabsorbed.
The simplest way to approximate the time it will take to reabsorb the anthropogenic flux is to calculate how long it would take for the atmosphere to revert to preindustrial levels of 280 parts per million if humans could cease emissions immediately. If the current net sink of around 4 gigatons of carbon per year remained constant over time, it would take about 50 years for the atmosphere to return to 280 ppm. However, there is no reason to think that these sinks would remain constant as emissions decrease. Indeed, it is more realistic to anticipate that the net sink would shrink in proportion to the decrease in emissions.
Scientists can approach this problem in a number of different ways. They can use models of carbon sink behavior based on their best knowledge of the physics of ocean carbon absorption and the biosphere. They can also use records of changes in atmospheric carbon dioxide during glacial periods in the distant past to estimate the time it takes for perturbations to settle out.
Using a combination of various methods, researchers have estimated that about 50 percent of the net anthropogenic pulse would be absorbed in the first 50 years, and about 70 percent in the first 100 years. Absorption by sinks slows dramatically after that, with an additional 10 percent or so being removed after 300 years and the remaining 20 percent lasting tens if not hundreds of thousands of years before being removed.
As University of Washington scientist David Archer explains, this “long tail” of absorption means that the mean lifetime of the pulse attributable to anthropogenic emissions is around 30,000 to 35,000 years.
|Figure via Global Warming Art.|