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Paris Climate Agreement: Beacon of Hope

Frequently Asked Questions

Questions below are a synopsis of what we've been asked when we've presented the research that has led to this book at various international meetings (American Geophysical Union; American Meteorological Society; European Geosciences Union), national laboratories (NASA Goddard Space Flight Center, National Center for Atmospheric Research), and universities (Columbia University, Colorado State University, Harvard University, Johns Hopkins University, Toronto University, University of Maryland).



What is your agenda?

This is a fairly frequent question and, since it is fairly loaded, we have chosen to place our answer to this question first.


Our agenda is simple: the truth, the whole truth, and nothing but the truth.

We're confident however this is how nearly all scientists would reply.

For what it is worth, let us add:

  • we did not set out to show the CMIP5 (Climate Model Intercomparison Project, Phase 5) GCMs (atmospheric, oceanic general circulation models) warm too quickly.  Rather, we stumbled across this result upon a fairly straight forward analysis of the climate record. We then devoted a major internal effort to first assessing and then shoring up this finding, by considering a full range of possible uncertainties in both our empirical evaluation of the human influence on global warming as well as our extraction of this signal from the CMIP5 climate models

  • we will derive no financial benefit from this book: the entire content is available free of charge to anyone in the world, via Open Access.  In return for arranging Open Access, we forfeited any royalties from the sale of the book. We hope the book sells many copies. If so, this shall financially benefit the book publisher and not the five co-authors

  • the salaries of the co-authors of this book have been supported by the selection of a peer reviewed proposal submitted to the NASA Climate Indicators and Data Products for Future National Climate Assessment (INCA) program as well as by the State of Maryland (i.e., two of us teach full time; the other three have taught part time).  The words in the book reflect only our views: we do not speak for NASA, the State of Maryland, our University, or anyone else

  • while we have spoken on this topic at various international meetings, national laboratories, and universities, we have never received an honorarium.  On rare occasion, a host has paid our travel cost. Usually our talks are given when a speaker happens to be in town for some other event, such as a scientific workshop

Why should we believe your climate model projections when so many others, such as those who worked on IPCC (2013), disagree?

This question, also fairly frequent, usually comes with a heap of scorn, a dose of ridicule, plus a topping of "you must not have read the IPCC report".


We are intimately familiar with the fourteen Chapter Physical Science Basis, 2013 report issued by the Intergovernmental Panel on Climate Change.

We teach from the report: the five co-authors have been involved with three courses at  the University of Maryland (Introduction to Weather and Climate; The Science, Governance, and Economics of Climate Change; Atmospheric Chemistry and Climate) that utilize material from various IPCC reports.

We invite those quick to assert that our findings disagree with the Physical Science Basis report of IPCC (2013) to have a careful look at Figures 11.25 and TS.14 of this document.  These figures show a trapezoid described by the authors of Chapter 11 and the Technical Summary as the indicative likely range of global mean surface temperature (GMST) during the period 20162035, based the IPCC author teams' expert assessment of the GCM output.

The IPCC trapezoid, shown in green on the figure above, is featured prominently in our book.  The projections of GMST found using our Empirical Model of Global Climate are in remarkably good agreement with the IPCC projection of how GMST will rise over the next several decades.

To suggest a fundamental difference between the EM-GC projections of global warming and the expert judgement of IPCC indicates a lack of familiarity with both Chapter 11 and the Technical Summary of IPCC (2013).

If the GCMs warm too quickly, what is wrong with these models?

It is hard for some to fathom that so many of the large climate models could be in error.  Generally, we are asked this question by colleagues who don't work directly with the development of these models.

As explained in Section 2.3 of our book, the most likely reason the CMIP5 GCMs warm too quickly is that climate feedback within these models is too large.  Climate feedback refers to responses within Earth's climate system that either amplify ("positive feedback") or dampen ("negative feedback") the initial perturbation, which in this case is warming driven by an increase in the radiative forcing of climate (RF) due to human release of greenhouse gases (GHGs).


As outlined in our book, cloud feedback tends to be positive in nearly all GCMs. It has proven to be quite challenging to define cloud feedback from observations.  There are major international efforts underway to improve both the empirical determination of cloud feedback (one of the book co-authors is presently serving on a US National Academy Panel focused on this and many other observational needs) as well as the evaluation and representation of cloud feedback within climate models.  Finally, we note that two of the 42 CMIP5 GCMs (INM-CM4, from the Institute for Numerical Mathematics, Russian Academy of Sciences and MRI-CGCM3 from the Meteorological Research Institute, Japan) we have examined exhibit a value of attributable, anthropogenic warming rate (AAWR) over the past three decades that is in very close quantitative agreement with the value of AAWR inferred from the climate record (see Table 2.3 of our book).  Our empirical model of global climate does exhibit characteristics similar to some of the established GCMs. The reason for the better agreement with the simulations of climate submitted to the CMIP5 archive by these groups is an area of active research.

The RCP 4.5 scenario has atmospheric levels of CH4 being nearly unchanged during the course of this century.  How does this complicate your identification of RCP 4.5 as the  2C pathway?

We are almost always asked about methane (CH4).

The future trajectory of atmospheric CH4 is vitally important.


Our climate modeling effort identifies RCP 4.5 as the 2C pathway and this is indeed complicated by the fact that the atmospheric mixing ratio of CH4 at the end of this century in RCP 4.5 is 1.6 parts per million (ppm), lower than the present value of about 1.84 ppm.


This complication is the focus of Section 4.4.2 of our book.  The production of electricity by the combustion of natural gas (which is mainly CH4) yields about 70% more power, per atmospheric CO2 molecule released, than the combustion of coal. Many countries are transitioning from coal to natural gas, driven by both economic factors (i.e., new abundant supplies of natural gas, such as the source supplied by fracking in the US) and air quality concerns (surface ozone and particulate loading, the pollutants that drive air quality, fare much better when the use of coal is reduced).


There are two difficulties with this transition.  First, if CH4 leaks to the atmosphere, the transition to natural gas can impose a climate penalty, rather than provide a climate benefit, since CH4 is a more potent GHG than CO2.  The image below, based on Figure 4.12 from our book, quantifies the probability of achieving the goal (1.5C) or upper limit (2.0C) of the Paris Climate Agreement as a function of the future evolution of CH4:



The second and perhaps more acute problem with the transition from coal to natural gas is related to the question of whether or not CH4 will truly be a transition fuel.  As shown on the summary page and detailed in Chapter 4, in order to place global emissions of GHGs on the RCP 4.5 pathway, half of the world's energy must be produced by sources that do not release GHGs by year 2060.  Electric Generation Units (EGUs) commissioned in the next few years will likely be designed with a lifetime that will extend past 2050.  Therefore, CH4 is vitally important for assessing how to achieve the goals of the Paris climate agreement.

Wasn't there a global warming hiatus?

This question is so "2014"!  But we include it here, nonetheless.  We had often been asked this question, of course before the recent ENSO-driven rise in global mean surface temperature.

The so-called Global Warming Hiatus is the subject of Section 2.4 of our book.  There had been considerable attention devoted to an apparent cessation of global warming, for analyses of temperature trends from the start of 1998 to the end of 2012.


To make a long story short, the skeptics of global warming seized upon this oddity, which was driven by a strong El Nio Southern Oscillation (ENSO) climate event that began in September 1997. This reorganization of the tropical Pacific Ocean adrove a rapid increase in global mean surface temperature in early 1998, the start of the hand-picked time interval for analysis of the hiatus.  Climate scientists "took the bait" and there has been an extensive amount of scholarly activity devoted to this topic, much of which is discussed in our book.


Our modeling efforts are in line with the interpretation first published by Karl et al. (2015) that there was never a true hiatus in the rise of global mean surface temperature. Rather, over the 1998 to 2012 time period, the rise in temperature was somewhat slower than had occurred previously (and has occurred since) due to two factors: the tendency of the climate system to be in a more La Nia-like state during the later half of this time interval, coupled with a smaller than normal rise of total solar irradiance during the period of peak activity of the most recent solar cycle (number 24 for those keep score).  Our Empirical Model of Global Climate provides remarkably good simulation of the rise in temperature over the time period of the so-called hiatus:


Your forecasts suggest global warming could stay beneath the Paris target of 1.5C, but the past two years have seen global average temperature almost at this level. Doesn't that mean your hypothesis can already be shown to be wrong?

We've gotten a similar question at a few recent presentations and we anticipate this will be a primary source of criticism on our contention that RCP 4.5 is the 2C pathway.

To quote the great Yogi Berra, "it is deja-vu all over again".


First we had the called Global Warming Hiatus, which was driven by the ENSO event of 1997-98.  Now we have a rapid intensification of global warming that is clearly linked to the ENSO event that begin around May 2015 (see for a nice animation of sea surface temperature), which has led to many news stories focused on how close we are to reaching the Paris Climate Agreement goal of 1.5C global warming.


Our Empirical Model of Global Climate accounts for the effect of ENSO.  We considered observations of global mean surface temperature from three of the leading data centers, until the end of 2015, in all of the published figures.


Past ENSO events show that global temperature will decline back to ENSO-neutral conditions within a year of the ENSO event.  Such a decline is readily apparent in the latest available data from CRU at the time of book release (i.e., Jan 2017):



Our global warming forecasts (red lines marked RCP 4.5 & RCP 8.5 above) assume neutral conditions for ENSO, major volcanoes, solar irradiance, as well as the Atlantic Meridional Overturning Circulation and the Pacific Decadal Oscillation. As such, since the end of 2015 was marked by anomalous warming driven by the ENSO event that began around May 2015, there is a bit of a "data shock" in some of the figures (i.e., Fig 2.19 and 2.20) shown in the book.  It just so happened we went to press in the middle of an extreme ENSO event.  Rather than try to model the precise timing of the recovery from ENSO, which as illustrated above can take many months, we instead focused on the use of radiative forcing of climate due to anthropogenic factors to drive our forecasts of global warming.

Would you bet the world that your model is correct?

We were asked this question when presenting at a session advocating for the divestment of Universities from investments in fossil fuels, at the Climate Action 2016 meeting.

Finally, an easy question to answer!


No, we would not bet the world our model is correct. In fact, we would not bet the world the sun will come up tomorrow.


After all, it is the world!!!  For all we know, someone has placed a device at the L1 point that will block sunlight from reaching the Earth tomorrow.


In all seriousness ... we are not suggesting that the climate projections found using our EM-GC replace those of the CMIP5 GCMs.  A huge amount of research effort has gone into the CMIP5 GCMs.  These models forecast a myriad of policy relevant climate variables such as precipitation, sea level rise, the state of the cryosphere, and conditions conducive to severe weather that our simple model does not address. Nonetheless, global mean surface temperature, which our model has been designed to forecast, has emerged as an important policy relevant metric upon which the Paris Climate Agreement will be assessed.

As stated in our book (Preface and Chapter 2):

We urge that judgement of our EM-GC projections of global warming be based on whether other research groups are able to reproduce these findings, using similar types of analyses. Given these caveats, our forecasts of global warming suggest that GHG emissions of RCP 4.5 constitute a reasonable guideline for attempting to achieve both the Paris target (1.5C) and upper limit (2.0C) for global warming, relative to the pre-industrial era. We urge decision makers to seek their own independent assessments of the veracity of all global warming projections being used to inform policy.


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This page last updated on Monday, 09 January 2017