Reducing greenhouse gas emissions may seem like taking responsibility for the sake of future generations. But the pace of climate change is certainly meaningful within a single lifetime.

One way to think about climate change, as explored in a new study led by Victoria University of Wellington’s Dave Frame, is that temperature patterns eventually move out of the range you’re accustomed to. Weather and climate are naturally variable, but if the climate shifts, unusual conditions can become the new normal. The “unusual” end of the spectrum gets replaced with more extreme conditions than before.

Defining the unknown

In this case, the researchers focus on the ratio of signal to noise—the warming change versus the normal range of variability. Specifically, starting with a bell curve distribution defined by the mean and the standard deviation, the researchers defined changes based on the average annual temperature by a standard deviation.

One standard deviation over, which they call “unusual,” there is still a 62-percent overlap between the new temperature distribution and the old one. Two notches over, we reach “unfamiliar,” where the new mean is warmer than 98 percent of the years that came before. At this point, the three coldest years in any decade would have been the three warmest years in previous decades.

Once the mean moves three standard deviations, the new mean is warmer than 99.9 percent of previous years—“unknown” territory.

These changes are obviously defined fairly locally, because they depend on both the rate of warming and the range of natural variability, both of which vary around the globe. The Arctic is warming much faster than the tropics, for example, but the tropics have a much smaller range of natural variability.

Who's noticing changes?

A useful way to break this down is to calculate the percentage of the Earth’s population crossing into “unusual,” “unfamiliar,” and “unknown” climates as the world warms. The researchers did this for several climate projections that used different scenarios of future greenhouse gas emissions. For each, they figured out how many people you can say would see the benefits of emissions cuts in their lifetime.

First up was a middle emissions scenario that results in about 2.4 degrees Celsius total warming by 2100. (For comparison, if every nation met the pledges they submitted for the Paris Agreement and carried that trend past the agreement’s end in 2030, we would see around 2.8 degrees Celsius warming.) Because the signal-to-noise ratio is high near the tropics, large populations in India, Malaysia, Indonesia, and western Africa mean that half the world’s people would experience “unknown” climates around the 2050s. By the end of the century, that would rise to 80 percent of the world’s population.

For the clearest picture of the value of reducing greenhouse gas emissions, the researchers then compared the highest and lowest emissions scenarios from the last IPCC report. The high, no-reductions scenario leads to almost 5 degrees Celsius warming by 2100, while the (fairly implausible at this point) aggressive-reductions scenario would keep warming below 2 degrees Celsius.

In the low emissions scenario, most people are saved from seeing “unknown” climates, topping out in the “unusual” or “unfamiliar” categories instead. But in the high emissions scenario, things go way beyond “unknown.” Remember that each category is a warming shift by one standard deviation—in this scenario, you’d need several additional categories. The large difference between the two scenarios is basically the potential impact of emissions cuts.

Notably, slicing up the analysis into different groups of countries—small island nations, southeast Asian nations, and the least and most economically developed countries—highlights the fact that the effects of climate change are not equally distributed. Many poorer and more vulnerable nations would see the greatest shifts in climate familiarity. Emissions cuts that slow climate change would have the most noticeable (by this measure) stabilizing effect in these areas.

The researchers write, “Our analysis shows that near-term mitigation initiatives can prevent many climates from becoming radically different from those experienced in the recent past, that such effects happen well within a human lifetime, and that this is especially true for those whose communities would otherwise change fastest. In other words, many of the long-term benefits of mitigation can be internalized by many people alive today.”

Nature Climate Change, 2017. DOI: 10.1038/NCLIMATE3297 (About DOIs).