Carbon dioxide acts as a "greenhouse gas," trapping energy radiating from the Earth, warming its surface in the process. The more carbon dioxide that enters the atmosphere, the more energy gets trapped at the surface, leading to climate change. Exacerbating the situation is the fact that carbon dioxide is "sticky" -- meaning that, once emitted into the atmosphere, it can stay there for centuries, contributing to climate change the whole time. This post explores the "sticky" nature of carbon dioxide in the atmosphere, and what this implies for how society responds to climate change.
To begin, it is important to understand how carbon cycles through the planet's oceans, landmasses, and atmosphere, as shown in the graphic from the Intergovernmental Panel on Climate Change (IPCC), below:
The constant cycling of carbon makes it is difficult to pin down exactly how long a given carbon dioxide molecule will stay in the atmosphere -- the best we can say is that a carbon dioxide molecule will stay in the atmosphere for 5-200 years on average. But even when carbon dioxide does make its way out of the atmosphere, Earth's natural systems can release other carbon dioxide molecules that were previously stored in the oceans/land back into the atmosphere, making the full effect of carbon dioxide emissions on surface temperatures much longer than this 5-200 year average.
The graphs, below, from the Global Carbon Project help to explain the impact of this carbon cycling in more detail. In particular, they show that, even with constant cycling through the Earth's ecosystems, nearly half of the carbon dioxide that humans have emitted since the start of the industrial revolution in the late 19th century have accumulated in the atmosphere.
And just how long does this fraction of our carbon emissions remain in the atmosphere? According to the IPCC, "the removal of human-emitted [carbon dioxide] from the atmosphere by natural processes will take a few hundred thousand years (high confidence).”
While a few hundred thousand years isn't much on a geologic time scale, its an eternity on a human one. So long, in fact, that scientists think of carbon emissions as a one-way street -- the planet's natural systems for cleaning the atmosphere of excess carbon emissions take too long to be considered relevant.
The fact that the planet cannot clean the atmosphere of excess carbon dioxide emissions quickly has significant implications for our fight against climate change. In particular, it means that we will have to both stop emitting carbon into the atmosphere and develop systems to clean up excess carbon that we emitted into the atmosphere in decades past to curtail climate change sufficiently.
To illustrate this point, scientists have devised the concept of a "carbon budget" which shows how much carbon dioxide we can emit into the atmosphere before significant climate change occurs. The graphic, below, from the UN Environment Program estimates that we only have about 1,000 gigatons (1 gigaton = 1 billion tons) of carbon dioxide left in our budget to "spend" before significant climate change occurs. And at our current pace of emitting around 50 gigatons of carbon dioxide annually, these scientists expect that we will stop emissions only after we have exhausted our remaining carbon budget.
Because the planet does not have a natural system capable of cleaning the atmosphere of excess carbon dioxide in a human-relevant timescale, it makes the development of solutions that hold the potential of removing and sequestering large volumes of carbon dioxide from the atmosphere a key priority if we want to avoid climate change. Because if we don't deploy these carbon removal solutions swiftly, the "sticky" nature of carbon dioxide in the atmosphere will mean that we will be stuck with climate change for a very long time.