The urgency of action on climate change is a complicated thing. On the one hand, the harm of inaction is real and growing every year. On the other hand, there's no such thing as "too late." Plug in some numbers, and you can define a corresponding deadline to hit a target, but we're not dealing with an all-or-nothing proposition. There is a continuum of consequences, and our choices can always move us one notch toward "better" or "worse."
With that in mind, the latest report from the Intergovernmental Panel on Climate Change (IPCC) shows the next few years are a critical window of opportunity for our hopes of limiting global warming to the benchmarks of 1.5° C or 2° C. Those numbers aren't magic, but they are meaningful. First, they represent better futures than any larger number on the thermometer. Second, these are the targets that international negotiations have long centered around.
(Giga)tons of work to do
The release is the third and final section of the 6th Assessment Report. The first two releases handled the physical science of a changing climate and the impacts of climate change. This one deals with climate solutions called "mitigation" in hazards parlance. The release examines past and present greenhouse gas emissions and illuminates the path to eliminating emissions and stabilizing our planet's climate.
There are clear signs of progress. Past reports were forced to describe future scenarios with massive emissions as "business-as-usual" continuations of current trends. This report says these scenarios no longer look likely. Rapid declines in the cost of renewables have led to accelerating growth of clean energy, among other trends. Together with national commitments, this puts us on a trajectory toward something like 3° C by 2100 rather than the 4+° C world of a "burn every bit of carbon you can find" emissions scenario.
Getting from there to a world that actually stops warming at 2° C or 1.5° C is a big job, though. The past few years of emissions data have raised the possibility that we stand near the peak of the emissions curve. That would have to be true. In scenarios where warming stops at about 1.5° C or by 2° C, the report says, "emissions are projected to peak between 2020 and at the latest before 2025." In other words, emissions have to start declining right about now.
And after the peak, emissions also have to steeply decline. For the 1.5° C limit, emissions have to drop more than 40 percent by 2030 and reach net zero in the 2050s. Hitting the 2° C limit isn't much easier—we must cut over 25 percent by 2030 and hit net-zero in the 2070s.
Another way to visualize these changes is to imagine a remaining "budget" of emissions before the world crosses a chosen limit. About 1,200 gigatons of CO2 would push us from where we are now to 2° C. Just 400 gigatons tons would push us to 1.5° C. For sobering context, the world emitted about 410 gigatons in the 2010s alone.
If you prefer something even less abstract, the report notes that "projected cumulative future CO2 emissions over the lifetime of existing and currently planned fossil fuel infrastructure" would be enough to warm the world to the 2° C marker. We'll have to intentionally shut down still-useful infrastructure to avoid exceeding climate goals.
Clear choices
In a press release, IPCC Chair Hoesung Lee said:
We have the tools and know-how required to limit warming. I am encouraged by climate action being taken in many countries. There are policies, regulations, and market instruments that are proving effective. If these are scaled up and applied more widely and equitably, they can support deep emissions reductions and stimulate innovation.
Lee is referring to actions that should be familiar by now, like replacing fossil fuel power plants with clean sources of electricity. Alongside that clean grid, electrification is key in the transportation, industrial, and home-heating sectors. Beyond fossil fuels, there are emissions from agriculture and deforestation to address. Much of this information is captured in a data-rich figure that shows both the potential of each of these solutions and their cost per ton of CO2 reduction. (Other gases are represented as an equivalent amount of CO2.)
"Mitigation options costing [US$100 per ton of CO2] or less could reduce global GHG emissions by at least half the 2019 level by 2030," the report says. That's a common threshold for cost-effective climate solutions, though most current carbon taxes are well under $100 per ton.
One notable addition to this report is a new chapter on "demand, services, and social aspects of mitigation". In addition to attacking emissions on the supply side (like a power plant), the report highlights the substantial opportunities on the demand side. This includes efficiency gains, urban planning, infrastructure for things like recycling or public transport, and behavioral shifts like diet and purchasing choices. Added together, the report finds, "Demand-side measures and new ways of end-use service provision can reduce global GHG emissions in end-use sectors by 40-70 percent by 2050 compared to baseline scenarios."
The different future-emissions scenarios in the report rely on varying amounts of carbon capture—taking CO2 from the air and sequestering it in another form. That includes forest expansion, soil management, industrial-scale facilities capturing CO2 through a chemical process, and the use of biofuels in power plants that capture CO2 and inject it underground. Some scenarios limiting warming to 2° C (for example) get there through ambitious carbon capture, while others involve more aggressive emissions cuts instead.
The 1.5° C scenarios include carbon dioxide removal equivalent to between one and 25 years's worth of current global emissions by 2100. The 2° C scenarios see between three and 20 years' worth. In any scenario where we successfully reach net-zero emissions, some amount of carbon capture is likely employed to offset the last and most difficult to solve emissions. (Hence the "net" in "net zero".)
Ultimately, the report says we need a lot more investment than we're currently putting in. "Average annual modeled investment requirements for 2020 to 2030 in scenarios that limit warming to 2° C or 1.5° C are a factor of three to six greater than current level," it says. We aren't waiting on the know-how—we're waiting on the resolve and the financing to get it done. That's particularly true for developing nations, which the wealthiest nations have repeatedly balked at assisting, despite the shared benefits.
In the IPCC's press release, Jim Skea (co-chair of the group behind this report) said, "It's now or never, if we want to limit global warming to 1.5° C (2.7° F). Without immediate and deep emissions reductions across all sectors, it will be impossible."
Failing that, the work to halt warming at 1.7° C, 2° C, or 2.3° C would be just as worthy. But depending on what happens in the next few years, the best futures on the list could start getting crossed off.
