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How global warming can be speeding up, slowing down, and going steady all at the same time

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Is global warming of the Earth’s surface speeding up, slowing down, or just coasting along? The answer is all of the above.

How can this be the case?

Changes in climate occur on different time scales. The question is always: what time interval is required for a meaningful signal of change to emerge versus what is considered irrelevant noise. Is it 15 years, 30 years, or even longer?

It depends on who you ask. Interestingly, I’ve found meteorologists – whose eyes are focused on the short-term (i.e. weather) – tend to make much out of trends observed over shorter time periods. Meanwhile, climate scientists – who take more of a long view – are more prone to dismiss short-term fluctuations as trivial. There’s a cultural difference in the lens through which some in these two disciplines view data and it has implications about the conclusions they draw about recent temperature trends.

Some meteorologists question claims of rapid global warming based on a so-called “pause” (or “hiatus”, “slow down”, “plateau”, or “deceleration”) in recent global-average temperatures.

Meanwhile, some climate scientists say the pause is to too short to be of statistical relevance. They call it a “faux pause” and point to other indicators of global warming such as heat stored in the deep ocean, sea level rise, and the melting of ice in the Arctic and Antarctic.

Both camps have data to support their arguments.

Examining trends in global warming averaged over periods of different length, there is evidence for acceleration (an increase in the rate of change) over longer time horizons and deceleration (a slow down in the rate of change) over shorter horizons. This is clear from the evolution in the trends in the rate of warming per decade in NOAA’s temperature dataset, for averaging periods of 15, 30 and 60 years (see below).

(Note: This analysis is solely intended to be illustrative, computing linear trends using one data set in a Microsoft Excel spreadsheet using its SLOPE function. Don’t mistake it as a rigorous exercise intended for publication in a scientific journal.)

On the one hand, the rate of warming using 15-year averaging periods (see the blue curve) has dropped sharply over the last several years and is currently as low as it’s been since the 1970s. That’s the short-term deceleration Capital Weather Gang meteorologist Matt Rogers wrote about.

On the other hand, the rate of warming using 60-year averaging periods (see the gray curve) has steadily climbed since the 1970s – that’s evidence of acceleration over the long haul.

Using 30-year averaging periods (see the orange curve), the rate of warming climbed from the 1960s to around 2000 (an acceleration), and has coasted over the past decade at a steady rate of around 0.16-0.18 degrees C per decade (which, incidentally, is very close to the near-term warming projections in the IPCC of around 0.2 degrees C per decade).

(As an aside, irrespective of the length of the averaging period used, the rate of warming has been positive since the 1970s. There is no “global cooling”.)

One thing which stands out in the evolution of the warming rates using the short 15-year averaging periods is their noisiness. They are prone to wild swings due to phenomena like El Nino, La Nina and volcanoes, which get smoothed out using longer averaging periods. If you focus too much on 15-year average data, you could easily miss or gloss over signals that manifest themselves over longer time spans. That’s the trap some meteorologists fall into and you could say was the case with Matt Rogers since he didn’t discuss 30-year or 60-year average warming rates in his controversial viewpoint. Not to mention, as Tom Di Liberto discussed in his blog post, the uncertainty in trends is greater for noisier data.

Before folks demonize meteorologists like Rogers for being “deniers”, I don’t think there’s anything wrong or nefarious about the broader point of his piece which is that IF the rate of surface warming does not pick up some point soon, it could well signify our expectations and computer model forecasts for global warming are on the high side (not fully accounting for natural variability) at least in the short-term.  Publishing climate scientists have said this much.  But, at the same time, I wouldn’t necessarily bet on a continuation of this short-term warming slow-down (or a transition from  a “faux pause” to a “true pause”).

After a period of La Nina-like conditions in the Pacific (which dampened surface warming), El Nino is likely on the way, which would inject a lot of heat from the tropical Pacific into the atmosphere. Not to mention the last two months were the warmest on record. In other words, the short-term slow down in surface warming in recent years could easily reverse itself. Such short-term changes are important to pay attention to as we monitor and try to better understand climate,  but must be viewed and interpreted within the context of longer-term changes.