Media channels have been flooded with news about the upcoming meeting in Paris of world leaders to discuss global climate change. The Obama White House is preparing to take a leadership role by issuing its final EPA rules for regulating greenhouse gas emissions. In preparation for any potential agreement in Paris, Entelligent asked Michael Mann to update investors on what we know about climate risk.


Much has been made of the supposed hiatus in global warming—what I have come to refer to as the “Faux Pause”, given the illusory nature of any short-term slowdown that occurred in the warming of Earth’s surface. This temporary slowdown was likely due to a combination of natural factors including a very small temporary downturn in the brightness of the Sun (as part of the 11 year solar cycle), the cooling effect of a fleeting uptick in global volcanic activity, and internal oscillations in the climate system related to the El Nino phenomenon.

Is the slowdown over? If so, what does it mean for near-term climate risks?

Earlier this year, my co-authors and I published an article in the journal Science showing that an internal oscillation in the climate system (which we term the “Pacific Multidecadal Oscillation” or just PMO) responsible for a tendency for cold “La Nina” conditions in the tropical Pacific during the past decade-and-a-half, likely offset some of the global surface warming during that time period (in part, by temporarily burying more heat below the ocean surface). Nature has ironically struck a Faustian bargain on our behalf, however, for, while the surface warmed less than it would have during that time period, there was an even faster buildup of heat in the ocean.

We noted that the PMO had reached peak negative values, corresponding to one of the most sustained intervals of La Nina-like conditions in the observational record (see chart). We infer that it is overdue to bottom out and then reverse, i.e. we are likely to soon move into a phase of more frequent and persistent El Nino events, and as a result, accelerated surface warming.

Almost as if to underscore the point, as our article went to press, the announcement came that 2014 had just beat out all previous contenders (both 2005 and 2010, albeit only narrowly) to establish itself as the warmest year on record globally. And 2015 is now off to the warmest start ever. The resurgent warmth is due in part to an unusually persistent El Nino event that, while remaining somewhat feeble, has nonetheless lingered for more than a year, ending the stranglehold that it’s cold cousin, La Nina, seemed to have on recent global climate. El Nino finally now appears to be hitting its stride. Current forecasts indicate that there is a very good chance the event will strengthen and persist through the winter.

This latest event could be the harbinger of a new area of more frequent El Ninos. What would that mean for climate risks? Answering that is a bit complicated. Global sea level actually tends to decrease during La Nina events and increase during El Nino events (this is due to the influence of El Nino on larger-scale weather patterns; El Ninos tend to dry out the tropical continents—that lost moisture ends up as seawater in the tropical oceans, increasing global sea level). So an El Nino-like climate era is likely to be one with accelerated overall sea level rise. Thus, it means heightened coastal risks, right?

Perhaps. But our worst Atlantic hurricane seasons [e.g. (1) 2005 with its unprecedented 27 named storms, 3 landfalling category 5 hurricanes including the devastating Hurricane Katrina and (2) 2012, with its 19 named storms and perhaps most damaging Atlantic storm in modern history, “Superstorm Sandy”] were each associated with a late season transition toward La Nina conditions. In fact, El Nino years tend to be associated with increased wind shear in the tropical Atlantic, something that is unfavorable for hurricane formation (this is the primary reason that my group at Penn State is forecasting a relatively inactive Atlantic hurricane season this year).

In fact, the discussion of hurricanes leads us to another matter. While the PMO may be about to reverse its down-trending behavior, another internal climate oscillation termed the Atlantic Multidecadal Oscillation (or “AMO”), which has been on the rise in recent decades, appears to have plateaued lately (see chart). This climate mode may soon begin a down-turn. The implications are important. The rise in the AMO in recent decades meant Atlantic Sea Surface Temperatures increased faster than they would have from global warming alone. Increased warmth in the main development region of the North Atlantic tends to lead to more frequent hurricanes. In fact, some experts argue that the critical variable is the “relative” warmth, i.e. the difference between the Atlantic warmth and the warmth elsewhere in the Northern Hemisphere (see e.g. my own group’s research on this).  This particular metric has been on the increase over the past couple decades as the AMO has been up-trending while the natural oscillation component in large-scale Northern Hemisphere temperatures (NMO) has instead been down-trending (see chart). That is likely part of the reason for the unusually high Atlantic hurricane activity during that time period. A down-turn in this metric could mitigate Atlantic hurricane activity in the decade ahead, acting to produce fewer and perhaps less intense Atlantic tropical storms and hurricanes.

Now, back to the PMO: A return to more El Nino-like conditions would have numerous other ramifications. As a general rule, rainfall in the southwestern U.S. (including California) tends to be less prevalent during La Nina years (though there is a fair amount of variability from one event to the next). So a return to El Nino-like conditions in the tropical Pacific could mean a respite from the devastating, unprecedented current California drought. Some scientists, in fact, have linked the California drought, at least partially, to the persistent La Nina conditions of the past decade, i.e. the very same factors associated with the Faux Pause. Yet that isn’t the whole story. Drought continues up through the present despite the transition more than a year ago to neutral or El Nino conditions. Other scientists (including myself) have argued that record heat has also contributed to the record California drought, a link which has often been ignored in other studies. Indeed, some scientists have linked (albeit tentatively) the very unusual winter jet stream pattern that has denied California much-needed rainfall in recent winters, while bathing Alaska in spring-like winter warmth, and chilling the eastern U.S. with sub-zero Arctic temperatures, to the rapid deterioration of sea ice in the Arctic (something that is directly attributable to global warming).

In the long-term, human-caused climate change poses distinct threats when it comes to water, food, coastlines, national security, and the health of our economy (something that has been duly recognized by led top economists and business leaders, hedge fund managers, and national security experts alike).  However, on shorter interannual-to-decadal timescales, there is substantial uncertainty in these impacts, in part because of the role of natural internal variability. Stakeholders and risk managers here in the U.S. must make decisions on the 5-10 year timeframe in the face of this uncertainty. To the extent we have some predictability on the 5-10 year time horizon due to the behavior of the PMO and AMO), we might speculate that hurricane risks to the U.S. east and Gulf coasts will be moderated in the decade ahead. Similarly, we might expect to see a short-term respite from the severe California drought of the past several years. In both cases, however, climate change is likely to worsen these impacts in the longer-term.


Michael Mann is Distinguished Professor of Meteorology at the Pennsylvania State University and author of The Hockey Stick and the Climate Wars: Dispatches From the Front Lines (with guest foreword by Bill Nye "The Science Guy") and Dire Predictions: Understanding Climate Change (updated 2nd edition now available). You can find him on Facebook and at twitter @MichaelEMann.