Can we limit global warming to 1.5°C? What would it require? Would there be real advantages compared to letting earth’s climate warm more than that? These are the questions that the Intergovernmental Panel on Climate Change (IPCC) Special Report 15, Global Warming of 1.5°C seeks to answer. IPCC is, of course, discussing human-induced global warming, not natural climate change. I will discuss their answer to the first question in this post, and the other two questions in the next post.
Let’s start by understanding what we can expect from this report.
Figure 1 shows an image of something. It appears to be something white. It is too far away and out of focus to see more. Figure 2 moves a little closer. Now it is possible to see that that it is a white rectangle with some gray smudges on it.
Figure 3 moves a little closer. You can’t see the whole of the white rectangle, but the gray smudges can now be seen to be a word: “Titanic.” But the writing is still out of focus. Figure 4 moves a little closer still, and the writing is now in clear focus.
Over the years, the IPCC has issued a series of reports on global warming/climate change. Over that time, the basic understanding of global warming has not changed. But as we have gotten closer, it has come more clearly into focus, and it has become possible to make out details that we couldn’t see before. We still don’t have global warming in full focus; we’re not to Figure 4 yet. But it has become possible to ask specific questions and give answers that, while not yet fully specific and detailed, are getting there. So, Global Warming of 1.5°C doesn’t contain radical new understandings. Rather, it is more detailed, and that is useful.
By the way, I chose the word “Titanic” on purpose. That ship was not built to survive a catastrophic iceberg strike, substandard steel may have been used to construct her, and she didn’t have enough lifeboats for all of the passengers. The captain denied the risk and sailed through the night into an iceberg field. By the time the iceberg was spotted dead ahead in the middle of the night, it was too late to turn and too late to stop. By that point, nothing they could do could change their fate: the Titanic was going to hit that berg and sink, and thousands were going to die.
Did I really write that? That’s really catastrophic, apocalyptic even! According to the IPCC report, we are very, very close to being like the Titanic. It may already be too late, but perhaps if we try really, really hard, it isn’t. Read on.
Figure 5. Global Mean Surface Temperature 1850-Present. Source: IPCC 2018.
Human activity has already caused our planet’s global mean surface air temperature (GMST) to warm approximately 1°C (1.8°F) since pre-industrial times, according to the report. GMST is increasing by about 0.2°C (0.36°F) per decade. The rate of warming appears to be increasing. Figure 5 shows the temperature trend. The gray line shows the monthly temperatures in the datasets. The orange line shows the change forced by both humans and nature combined, while the yellow line shows the change forced by human activities alone (it is hard to see because it is embedded in the yellow band, look closely) .
GMST is an average across the globe. Some regions have warmed more than others. For instance, the temperature over land has increased more than the temperature over water; 40-60% of human population lives in regions that have already warmed 1.5°C (2.7°F) or more. Thus, a 1.5°C increase in GMST implies a larger than 1.5°C increase over land, with a smaller increase over the ocean.
Past emissions (through 2017) are probably not sufficient to cause GMST to increase more than 1.5°C. Therefore, warming limited to 1.5°C is theoretically possible if human emissions are immediately reduced. Two ways in which the 1.5°C limit could be achieved are discussed in the report. One reduces GHG emissions sufficiently quickly so that the 1.5°C limit is never exceeded. The other would allow a small overshoot of the limit, with temperature then being brought back within the limit by removing carbon dioxide from the atmosphere.
Figure 6. GHG Emission Reduction Pathways. Source: IPCC 2018.
To limit the increase of GMST to 1.5°C with no overshoot would require GHG emissions of no more than 25-30 billion metric tons of CO2e per year in 2030 (compared to estimates that under business as usual they will be 50-58 billion metric tons per year). And GHG EMISSIONS WOULD NEED TO DECLINE TO NET ZERO BY 2050. That’s right – no net GHG emissions by 2050. Figure 6 shows the reductions over time in emissions of CO2, methane, black carbon (soot), and nitrous oxide consistent with a 1.5°C increase in GMST.
The no-net-emissions requirement could be met by two strategies: the first would involve reducing emissions themselves. Reducing emissions at this magnitude would require near-total transformations of our energy, transportation, and agricultural systems. The second would involve widely deploying carbon dioxide removal mechanisms. The only currently proven mechanism for removing carbon dioxide from the atmosphere is revegetation, especially reforestation. Attempts to add carbon capture and sequestration to power plants have not yet proven viable.
The limits agreed to in the Paris Climate Agreement are not sufficient to limit the increase in GMST to 1.5°C.
In the next post, I will look at what the report has to say about strategies to meet the limit, and what the costs and benefits might be.
Sources:
Intergovernmental Panel on Climate Change. 2018. Global Warming of 1.5°C (Draft). Downloaded 11/24/2018 from https://www.ipcc.ch/report/sr15.
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