Significant acceleration of global heating since 2015

 From the Potsdam Climate Institute

Global warming has accelerated since 2015, according to a new study by the Potsdam Institute for Climate Impact Research (PIK). After accounting for known natural influences on global temperature, the research team detected a statistically significant acceleration of the warming trend for the first time. Over the past ten years, the estimated warming rate has been around 0.35°C per decade, depending on the dataset, compared with just under 0.2°C per decade on average from 1970 to 2015. This recent rate is higher than in any previous decade since the beginning of instrumental records in 1880.

Global warming rate (in °C per decade) from the Berkeley Earth global temperature data: The blue line shows the linear trends for the time before and after 2015 (light blue the uncertainty range). The red line shows the linear trend for 10‐year windows of the data, at 1-year intervals. Figure: PIK

“We can now demonstrate a strong and statistically significant acceleration of global warming since around 2015,” says Grant Foster, a US statistics expert and co-author of the study, which was published today in the scientific journal Geophysical Research Letters. 
 
“We filter out known natural influences in the observational data, so that the ‘noise’ is reduced, making the underlying long-term warming signal more clearly visible,” Foster added.
 
Short-term natural fluctuations in global temperature caused by El Niño, volcanic eruptions, and solar cycles can mask changes in the long-term rate of warming. In their data analysis, which is based on measurement data, the two researchers work with five large, established global temperature data sets (NASA, NOAA, HadCRUT, Berkeley Earth, ERA5).
 
“The adjusted data show an acceleration of global warming since 2015 with a statistical certainty of over 98 percent, consistent across all data sets examined and independent of the analysis method chosen,” explains Stefan Rahmstorf, PIK researcher and lead author of the study. 

After correcting for the effects of El Niño and the solar maximum, 2023 and 2024, which were exceptionally warm years, become somewhat cooler, but remain the two warmest years since the beginning of instrumental records. In all datasets, the acceleration begins to become apparent in 2013 or 2014. To test whether the warming rate has changed since the 1970s, the research team applied two statistical approaches: a quadratic trend analysis and a piecewise linear model that objectively determines the timing of any change in the warming rate.

The study does not investigate the specific causes of the observed acceleration. However, climate models show that an increasing rate of warming is fundamentally within the scope of current climate modelling, according to the authors.
 
“If the warming rate of the past 10 years continues, it would lead to a long-term exceedance of the 1.5° limit of the Paris Agreement before 2030,” says Stefan Rahmstorf. “How quickly the Earth continues to warm ultimately depends on how rapidly we reduce global CO₂ emissions from fossil fuels to zero."

We face catastrophe unless we cut emissions.  We should replace coal and gas with wind, solar and batteries as fast as we can.  We should stop the sale of petrol (gasoline)/diesel cars, lorries and buses now, because it will take 15 years for the existing vehicle fleet to be replaced.  And before you fall to the floor and start chewing the carpet, there are electric alternatives to every petrol or diesel car from small city cars, through utes (pick-up trucks), light vans through semi-trailers and buses.  We need to replace all oil/gas heating with heat pumps.  Is mankind too frigging feckless to act?

Global warming *has* accelerated

 From Open Mind (Tamino)

I won't repeat the whole article.  Read it in full here.

My paper with Stefan Rahmstorf showing that global warming has accelerated was published in Geophysical Research Letters today. The main result is that global warming is NOT proceeding at the same old rate it has been since 1975. It’s going faster.

In this data set [from Berkeley Earth], 2025 turned out to be the 3rd-warmest year on record (as in the other data sets except NASA, where it came in 2nd). When I adjust the data to remove the estimated impact of el Niño, volcanic eruptions, and solar variation, I get this:

The trend is unaffected, but the noise level is much reduced, which enables us to estimate warming rates with less uncertainty. I’ve added a red line to this graph which is a modified LOWESS smooth of the adjusted data.

To test for acceleration we isolated the data since 1975, and the simplest way to test for it is to fit a parabola to the data; if the quadratic term is statistically significant, we can reject the null hypothesis, that the signal is just a straight line. Of course we must correct for autocorrelation of the noise, but still the quadratic term turns out to be strongly significant. We can safely reject the null hypothesis: there has been acceleration.

According to this model, the warming rate right now is the slope at the endpoint of the parabola, which is 0.28 ± 0.05 °C per decade (i.e. between 0.23 and 0.33 °C per decade, 95% CI). I will emphasize that this is the “best estimate” and those are the correct uncertainty levels IF (and this is a BIG IF) the data actually follow a parabola plus stationary noise. If not (which is the overwhelmingly likely case), we can consider the estimate good but not best, and the uncertainty levels are a lower bound on the actual uncertainty.

Another test for acceleration is to find the best fit of a continuous piecewise-linear function which is allowed to change slope at a time chosen by changepoint analysis. This is a challenge to evaluate statistically because we have to allow for autocorrelation and account for the extra degree of freedom to choose the changepoint time. But it can be done, and the best-fit model again turns out to be strongly statistically significant.

Both those models serve excellently to demonstrate the presence of acceleration. But I doubt they are best to estimate what the warming rate is right now, and what it will be in the near future. For that, I offer yet another model, which I will apply to the data since 1880, a continuous piece-wise linear fit (PLF) which is allowed to change its slope every 15 years from 1905 through 2010. I call this model “PLF15”

The PLF15 model not only estimates the signal value, it conveniently gives us an estimate of the average warming rate over each segment between the knots. I can plot the warming rate itself (which for this model is constant during each segment) along with light blue shading to show the uncertainty range.

All these graphs plot the warming rate in °C per year, but when quoting numbers I have followed the custom these days to talk about the rate in °C per decade. According to this analysis, the current estimated rate is 0.31 ± 0.07 °C/decade.

Which estimate is best? I don’t know, but I do know that even 0.24 °C per decade will take us past 2 °C right around the year 2050. The whole point of the Paris agreement is: DON’T GO THERE. My advice: fasten your seat belt, things are going to get ugly.

We need to redouble efforts to cut emissions, or things will get very ugly.  What can we do?

• Set a renewable energy target in every country.  The percentage of renewables+nuclear needs to rise by 6-8% a year, at least.  This will cut emissions by 27% (emissions from electricity generation are +-30% of total global emissions) within a decade.  We may not yet be able to go above 90 or 95% renewables in the grid, because we don't have long-term storage to offset periods of dunkelflaute, but we will have cut most emissions from electricity generation.  
• We must tax imports from countries which do not have an R.E.T. or a price on carbon.  (See my posts on a carbon border tax)
• We need to accelerate the replacement of petrol/diesel vehicles (ICEVs) with EVs.  This is a problem, because even when we reach 100% of sales being EVs, it will take 10-20 years for all ICEVs on the roads to be replaced.  This is too long.  Most countries are nowhere near 100% EV sales.   We could, for example, ban the import of new or second-hand ICEVs, or slap 100% taxes on them.   Ethiopia has already done this.  This policy should apply to two and three-wheeled vehicles, too.  Countries which do this deserve reduced carbon border taxes.
• In rich countries, we need to replace oil- or gas-based household and industrial heating with heat pumps.  Because they have high up-front costs, they will require government subsidy to start the revolution rolling.

All of these combined will cut emissions by 50 to 60%.  If we also switch to low-emission steel and cement, the emissions cuts could reach 70%.

That will leave (mostly) agriculture.  Put that in the too-hard basket for now--people love their meat too much to give it up.  But it won't go away.  When we've cut emissions by 70%, agriculture will dominate what's left over.  And action will no longer be postponable.

Our planet is warming twice as fast as we thought

 A video from Just Have a Think.  He discusses a paper by Grant Foster and Stefan Ramsdorf, which confirms that the rate of global warming has accelerated from 0.2 degrees C per decade to 0.4 degrees C.   One of the authors of this paper is Grant Foster, who is "Tamino'", and I've commented on his pieces published on his blog "Open Mind" several times over the last few years.  See this, and this, and this.

Just Have A Think  explains quite well the techniques used to prove their point.  He cautions that just because temperatures have been rising twice as fast as they were between 1970 and 2010 over the last decade, it doesn't mean that they will necessarily continue to do that.  

On the other hand they might, and they might even accelerate.  Just being prudent and cautious would suggest that we should accelerate our attempts to slash emissions, especially since wind and solar power and batteries and EVs have fallen so much in cost. 

A chart from the paper referred to in the video, showing the unadjusted and the adjusted (for volcanic eruptions, El Niño, and the sunspot cycle) time series from 5 different estimates of the average global temeperature anomaly.