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Net-zero emissions – But no cooling down without Negative Emissions Technology

Commitments towards moving to net-zero carbon emissions formed a critical part of the Paris Agreement. Limiting global warming to 2oC requires that the world reach net-zero CO2 emissions by about 2075. Limiting it to 1.5oC requires reaching net-zero by 2050. However, reaching either of these goals envisions that we continue living with a hotter planet. After net-zero is achieved, temperatures will still rise due to climate lag, and cooling the world will require Negative Emissions Technologies (NETs).  

Atmospheric temperatures play catchup 

Unfortunately, there is a lag between stopping carbon emissions and stopping temperature increases. Climate modeling (Meehl, et al.) indicates that if we slammed on the emissions brakes today, we would still get another half-degree temperature rise due to climate lag. The climate lag effect ensures that the warming continues for at least another 40 years after we reach our emissions goal. 

Climate lag implies that our net-zero goals are slightly deceptive. We have already seen a 1oC temperature rise. So if we hit net-zero emissions today, then we would limit the ultimate temperature rise to 1.5oC. If we reach that goal in 2075, we will then limit it to 2.5oC, not 2oC. The only way to fight this systemic momentum towards continued warming after reaching net-zero emissions is via Negative Emissions Technologies. But NETs represent nascent and unproven technology, yet to be applied at a global scale.

Natural feedback loops

NETs may also be required to counter the effects of positive feedback loops that contribute to increasing amounts of atmospheric carbon. Arctic Amplification and permafrost melt are cases in point. Arctic amplification is a process that involves temperature feedback from solar radiation. 

Sea ice has a high albedo and reflects a lot of solar radiation into space.  The darker ocean water is not nearly as reflective as ice, and it absorbs heat from solar radiation more efficiently than ice. Less sea-ice cover equals less reflected solar radiation and increased ocean warming. Warmer ocean water then warms the atmosphere.  

The Arctic’s summer ice cover has decreased by 40% over the past 40 years. A reduced ice cover results in more heat absorbed by the ocean, less ice as winter starts, and where ice does remain, it is thinner. These conditions all inhibit the formation of thick winter ice. So when summer returns, the winter ice melts more quickly, and then even more ice cover is lost.  If this cycle continues long enough, summer sea ice will disappear completely.

Reaching net-zero won’t immediately stop this process. The feedback loop is primarily dependent on sunlight and existing ambient temperatures.  The extra heat absorbed in the Arctic Ocean will further warm the Arctic and therefore initiate more permafrost melt.

Permafrost and greenhouse gases

Permafrost formed over centuries. During that time, plants and animals lived and died with their remains frozen into the soil’s structure before complete decomposition set in. The standard carbon cycle was interrupted by this process. Plants and animals require carbon to live, and they sequester that carbon in their bodies. Under most circumstances, their death starts a process where bacteria break down the organic material and release carbon back into the atmosphere. Permafrost creates a time capsule where decomposition is interrupted, and the carbon remains sequestered in the icy soil.

An estimated 1,400 gigatons of carbon lies frozen in the Arctic permafrost. By comparison, the earth’s atmosphere contains only 850 gigatons of carbon. 

As the permafrost thaws, it develops a landscape referred to as thermokarst topography. When warmed, the soil’s high ice content turns to water and promotes rapid erosion and collapse of the land surface. This land collapse results in shallow lakes, mud bogs, and mud mounds. Once unfrozen, the organic matter in the soil becomes food for bacteria. Then the bacteria rapidly consume the centuries-old organic materials and return the carbon to the atmosphere as carbon dioxide and methane.

Since the Arctic holds twice as much carbon as the atmosphere, the potential exists for significant releases of greenhouse gases from thawing permafrost. 

Even when we reach net-zero emissions, Arctic amplification and melting permafrost will keep temperatures rising and pump massive amounts of greenhouse gases into the atmosphere. The only way to counteract this is through NETs, a technology that is still in its infancy.

 Stopping carbon emissions doesn’t necessarily equate to stopping temperature rise.


Arctic Warming (Source: ArcheanWeb) –  Also:

Earth’s heat budget and global warming (Source: ArcheanWeb) –  Also:

A dose of realism in environmental research (Source: ArcheanWeb) –  Also:


What Does “Net-Zero Emissions” Mean? 6 Common Questions, Answered (By Kelly Levin and Chantal Davis; World Resources Institute) –  Also:

Guest post: The problem with net-zero emissions targets (By Prof Duncan McLaren; Carbon Brief) –  Also:

If we stopped emitting greenhouse gases right now, would we stop climate change? (Source: The Conversation) –  Also

How Much More Global Warming and Sea Level Rise? (By Gerald A. Meehl*, Warren M. Washington, William D. Collins, Julie M. Arblaster, Aixue Hu, Lawrence E. Buja, Warren G. Strand, Haiyan Teng; Science) – Also:

Climate Change: The 40 Year Delay Between Cause and Effect (By Alan Marshall; Skeptical Science) – Also:

Arctic amplification (Source: National Snow and Ice Data Center) –  Also:

Feature Image: Mosaic of the Arctic (Modified) – By NASA – NASA, Public Domain,  

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.