On The New WMO "Annual to Decadal" Climate Report

The World Meteorological Organization ("WMO") has just put out their latest "Global Annual to Decadal Climate Update".
Among their main predictions are:
Annual mean global (land and sea) mean near-surface temperature is likely to be at least 1°C warmer than pre-industrial levels in each of the coming 5 years
• The chance of at least one year exceeding the current warmest year, 2016, in the next five years is 90%
• Over 2021-2025, almost all regions, except parts of the southern oceans and the North Atlantic are likely to be warmer than the recent past (defined as the 1981-2010 average)
• Over 2021-2025 there is an increased chance of more tropical cyclones in the Atlantic compared to the recent past

Most of these predictions are straightforward and very likely to be true. The continued human-caused warming trend, due to ongoing fossil fuel burning and carbon emissions, makes record global temperatures and the likelihood of exceeding new thresholds (e.g. 1.5C warming) and "warm streaks" increasingly common with each passing year (see some of our own work on this).  Given that the warming trend is already at about ~1.2C (relative to pre-industrial) now and there is year-to-year variability of several tenths of a degree C about the warming trend, it’s almost certain that we’ll temporarily breach the 1.5C threshold sometime within the next few years. 
But there is huge potential for misunderstanding here, particularly when it comes to avoiding dangerous planetary warming thresholds, one of the most often-cited of which is 1.5C relative to the preindustrial. When we talk about the need to avoid 1.5C global warming in a climate change context, we’re talking about the long-term trend, not the values for individual years. Those will fall above that threshold well before the trend line crosses it, due to natural year-to-year variability. What we’re concerned about, when it comes to climate change impacts, is when the trend line crosses 1.5C, and that likely won’t happen for decades (indeed it can still likely be avoided given rapid enough reductions in carbon emissions). 
Another matter worth commenting on further is the prediction of warm ocean temperatures in the North Atlantic and high levels of Atlantic hurricane activity such as we've seen in recent years. Like the predicted record global temperatures, these predictions too follow from human-caused warming. Our own group predicted an unusually active hurricane season last year (as many as 24 named storms). This came closest of all the predictions to the actual observed total, but even our forecast underestimated last year's record total of 30 named storms. Our forecast of an active season was driven primarily by very warm tropical Atlantic ocean temperature, and it beat  (see discussions here and here) forecasts by other groups that were based, at least in part, on the notion of a long-term natural, internally-generated oscillation in North Atlantic sea surface temperatures sometimes called the "Atlantic Multidecadal Oscillation" or "AMO" (or sometimes called "AMV").
The very term "AMO" was, in fact, coined by none other than yours truly, more than two decades ago [see a discussion about this today in the New York Times (scroll down to "Climate Science that Didn't Hold Up" by Times science journalist John Schwartz)]. In more recent work we have shown that the "AMO" is likely an artifact, a consequence of competing human warming (greenhouse gas) and cooling (sulphate aerosol pollution) factors that masquerades as an apparent "oscillation" (see this previous post by me and our recent articles in Science and Nature Communications upon which it is based). There is, in fact, no compelling evidence, based on either observations or current generation climate models, for an internal multidecadal "AMO" climate oscillation.
The new WMO report unfortunately appeals to the flawed, out-dated notion that the current North Atlantic warmth and heightened Atlantic hurricane activity is a consequence of a temporary warm phase of an internal climate oscillation (they state: "Since the mid-1990s the North Atlantic Ocean has been in a warm phase of Atlantic Multidecadal Variability"), and predict a reversal in the future. Such predictions are poorly premised and, unsurprisingly, have a notoriously bad track record. Most, if not all, of the predictability instead appears to be associated, as noted above, with the response of the climate system to human and natural external drivers, not internal variability (which appears indistinguishable from noise).
A comprehensive recent review article in Nature Reviews on decadal climate predictability by Jerry Meehl an a dozen other experts (disclosure: I was one of the co-authors) concluded that there is no such evidence of a predictable "AMO"-like multidecadal oscillation:

Moreover, it is also difficult to objectively separate forced (natural and anthropogenic) and internal decadal to multi- decadal climate variability, adding further challenges for S2D prediction verification and triggering debate on best practices for signal separation (45–48).


However, there was no convincing evidence across these state-of-the-art coupled models for distinct oscillatory signals, other than on the interannual (years 3–7) ENSO timescales (179). These observations suggest, as noted previously, that low frequency variability on interdecadal timescales is characterized by broadband rather than oscillatory behaviour.

In summary, while new WMO report provides quite a bit of useful and important information, the WMO would be best served by focusing public attention on areas where there is consensus (i.e. on the warming of the planet caused by ongoing carbon pollution) and move away from more questionable predictions based on outdated concepts like the "Atlantic Multidecadal Oscillation". Emphasizing speculative and arguably dubious science is misleading to stakeholders and risk managers and it detracts from the key overall message of the report--that the record warming of the planet we are witnessing (and its impacts) are a manifestation of human-caused climate change and our ongoing burning of fossil fuels. On the very day that a court in the Hague has ordered the world's 4th largest oil company, Royal Dutch Shell, to cut carbon emissions in half within the next decade, that would seem to be the most relevant message to communicate to the public.


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