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.

https://arstechnica.com/?p=1846017