The hottest days in 120,000 years
Last week we set a record: we experienced the hottest day in the observational record. That record stood for 24 hours, when the next day took the record. In fact, these may have been the hottest days in the last 120,000 years.
To understand this latter claim, let’s work backwards. We know that the record breaking days last week are the hottest in the observational record, which goes back about 150 years.
Now, let’s look at the last 10,000 years. We used to think that the period about 7,000 years ago, a period sometimes referred to as the Holocene Optimum, was warmer. However, scientists have revised their estimates of the temperature at that time and it is probably the case that temperatures today are warmer than anytime over the last 10,000 years.
Beyond 10,000 years, we were in an ice age, so last week was clearly warmer than that. That ice age extended back to around 120,000 years ago, when the previous interglacial ended. That interglacial was a few degrees warmer than today, due to differences in the Earth’s orbit.
Voila, this week likely had the hottest days in the last 120,000 years. The word likely here indicates our imprecise knowledge of temperatures before the instrumental record. For example, the proxies used to estimate temperatures over the last 10,000 years are century scale, so we can’t rule out a single crazy hot day as a result of an exceptionally large El Nino or some other driver of variability.
Is this important?
Considering the entire history of the Earth, it has been both much warmer and much colder than today. So the fact that the Earth is warmer today than anytime over the last 120,000 years is not really important — other than for shock value.
What is important is that the planet is warmer than it was 30, 40, 50, 100 years ago. That matters because much of the infrastructure that we rely on was built in the last century — and for a climate that no longer exists. If you live in a city that has a sewer system that was built in the middle of the 20th century, then that sewer system was not designed for the intense rain events that we’re getting today. Similarly, if you live in a city on the coast, then it was built for a sea level that no longer exists.
We can update our infrastructure for the climate of the 21st century, or course, but it’s going to be an absolute nightmare.
Absolute temperature vs. anomalies
One interesting thing is that this statistic is based on absolute temperatures:
The data clearly shows a seasonal cycle, with global temperatures during the northern hemisphere's summer higher than the southern hemisphere's summer. This difference occurs because the northern hemisphere has more landmass, which heats up more quickly than the oceans.
This is an unusual way of looking at the problem. Most climate data you see are anomalies: the difference between the absolute temperature and the average temperature of a reference period. Here’s what the temperatures look like in anomaly space:
In anomaly space, last week was not a record. The reason is that the seasonal cycle is removed when the anomaly is calculated, so July temperatures do not get the boost from the seasonal cycle that they do when looking at absolute temperatures. In anomaly space, the hottest days occurred last Fall. Ultimately, it’s a nit: either the hottest temperatures were a either few days ago or a few months ago, but in either case humanity is in uncharted territory.
If you want to know more about anomalies, or if you’re curious why we use anomalies rather than absolute temperatures, you’re in luck! I put together a video for my class on this very subject.
Bonus content
I did an interview with Fox Live Now last week about this record.
Update: see this thread on mastodon from Allegra LeGrande, a scientist at NASA GISS, who’s looked into this in more detail.
It seems to me that a few important points are relevant.
One is that the current anomalously high temperatures are definitely related to the 2023-2024 El Niño oscillation. As with the El Niño oscillation of 1997-1998, we experienced record temperatures in 1998, followed by cooler temperatures for several years prior to experiencing the 1998 record being broken in the mid- and latter-half of the 2010s.
If El Niño is an important contributor to our current, anomalously high temperatures (along with greenhouse warming), we should expect cooler temperatures soon, for a least the next few years. However, James Hansen and colleagues have also identified reduction in sulfur emissions from maritime shipping as a factor, so perhaps we won't see the same sort of "pause" as we did after 1998.
Another point is that we should balance our discussions on items of "shock value" with info that expresses hope. For example, thanks to the development and commercialization of low-carbon and zero-carbon technologies, that are now economically-competitive with fossil energy technologies, worldwide anthropogenic CO2 emissions are no longer increasing at an exponential rate. And we certainly seem to have departed from the IPCC's "business-as-usual" pathway. In fact, if the International Energy Agency (IEA) projections are correct, we will see peak anthropogenic CO2 emissions during this decade.
The IEA, in noting that we are "bending the emissions curve", attributes this to the development of mass-producible, modular energy technologies like solar photovoltaics, wind turbines, heat pumps, batteries and electric vehicles -- which become cheaper through assembly line mass production. That is, economies of hardware mass production, at high enough mass production rates, are able to beat the traditional economies of scale that we previously achieved for large power plants and chemical plants in the 20th Century.
We can expect the same with fuel cells and hydrogen, the latter of which can be produced from water and biomass, with high efficiencies, using distributed, mass-producible, modular technologies as well.
These facts make it clear that the issues today aren't just political. They are also technoeconomic.
But the real bottom line is that, after flattening annual anthropogenic CO2 emissions, economic forces will naturally drive the continued development and adoption of low-carbon and zero-carbon energy technologies, so the likely outcome is continued worldwide reductions in annual CO2 emissions.
All of this brings me to a final point. The 2015 Paris Agreement, in targeting peak temperature increases of no more than 2 deg C, and still more preferably at 1.5 deg C, calls for the achievement of a) halting the rise in annual anthropogenic CO2 emissions and b) reaching a balance between anthropogenic emissions and removals by sinks, and calls for the latter of these to take place during the second half of the 21st Century.
Certainly, the IEA projection says that we are on route to achieving a) within a few years. In addition, even modest decreases in emissions each year after that will lead us to b).
By my calculations, taking into account the continued performance of the natural carbon sinks, we could hit peak CO2 concentrations in the neighborhood of ~500 ppm (or less), perhaps within a few decades, with declining values shortly after.
How quickly we achieve the balance between emissions and sinks depends on how aggressively we act, so this is a time to use our continuing success as evidence that we can do this -- in fact, we are already doing this -- and can get control of the greenhouse gas content of our atmosphere. This is certainly not a time for negativity or hopelessness!
Another excellent explainer post.