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Paper questionably assumes no add'l feedback to warming will occur, drops last five years of ocean data

Markus Huber and Reto Knutti, researchers at the ETH Zurich's Institute for Atmospheric and Climate Science, earlier this month published an interesting research letter in the peer-reviewed journal Nature Geoscience, entitled "Anthropogenic and natural warming inferred from changes in Earth’s energy balance".  This letter is significant, as it appears to be the first climatology work to try to extensively model warming from an energy balance perspective.  This represents a major step forward in trying to understand a model global warming.

Ultimately, the paper tries to assign a percent blame to mankind for the current warming.  However, there is good cause to debate the validity of these final conclusions, given some of the paper's rather naïve methodology.  It is here that the paper falls back on the mistakes of some of its predecessors in perhaps oversimplifying the system.

I. Mankind is Contributing to Warming the Earth

Whether it's one trillionth of a degree Celsius or one degree Celsius, there's one thing most scientific-minded observers of the warming debate can agree upon -- mankind is responsible for some part of the Earth's recent warming trend.

Basic physics tells us carbon dioxide is a greenhouse gas, as is methane.  Mankind has emitted a lot of carbon dioxide and methane in the last 100 years, and atmospheric monitoring has shown a discrete rise in levels of these greenhouse gases.

The greenhouse gases trap heat.  So mankind is clearly responsible for some quantifiable amount of the warming effect.

Energy balance
Global warming is symptomatic of either an excess incoming energy flow or an deficient outoging energy flow, thus the energy balance is a good means of examining warming.
[Image Source: Learner.org]

The important questions are:
  1. How much of recent warming (%) is mankind responsible for?
  2. How far can the system go before reaching equilibrium?
It is here, though, that the logical "skeptic" and some climatologists diverge.  Some researchers claim that both of these questions have been, more or less, definitively answered.  Scientifically-grounded skeptics, on the other hand, argue that much of the current work has been simply in fitting weak models to data sets, leading to potentially misleading conclusions.

The new work, for its part, does do an admirable job pointing out some of the shortcomings of the current modeling approach.  The authors write:

The optimal fingerprint detection and attribution framework provides a rigorous, statistical method to quantify the contributions of different external forcings and internal variability to the observed climate changes. In essence, it is based on a regression of the observations onto model simulated patterns and relies on the spatio-temporal response patterns from different forcings being clearly distinct. The assumptions are that climate models simulate the spatial patterns reasonably well and that regional responses from different forcings can be scaled and combined linearly. The global energy budget is not necessarily conserved and observed changes in the energy budget are not considered. Previous studies showed that observed patterns of surface air temperature provide a constraint on the human contribution to the observed warming.

In other words, past studies have taken temperature observations, assumed that we know exactly how much temperature increase is caused per unit forcing (e.g. per unit CO2), and linearly combined forcings to find a model that fits current temperature rises.

Forcing inputs
The new work attempts to combine forcings linearly and assumes no additional feedback, much like past works. [Image Source: ETH Zurich]

II. Looking at the Energy Balance

The new work alters this approach slightly, by using the energy balance, rather than the temperature, as the target to fit to.  But ultimately its conclusions may be skewed by the fact that it, like its temperature-based predecessor, uses a simplistic approach in which inputs' contributions:
  1. Scale linearly
  2. Are additive
  3. Are of equal efficacy (from the paper: "We assume that all forcing agents have equal efficacy.")
This is a rather simplistic model, although the analysis was complex with "thousands of model simulations".

The paper's ultimate conclusion is that:

Our results show that it is extremely likely that at least 74% (±12%, 1σ) of the observed warming since 1950 was caused by radiative forcings, and less than 26% (12%) by unforced internal variability. Of the forced signal during that particular period, 102% (90-116%) is due to anthropogenic and 1% (-10 to 13%) due to natural forcing.

The authors conclude that man is responsible for 74 percent of warming.

Temperature trends
The paper shows that the model was fit well with recent historic temperatures.
[Image Source: ETH Zurich]

The blame
The paper claims that mankind is responsible for 74 percent of warming via greenhouse gases,once the offsetting contribution of aerosols is considered. [Image Source: ETH Zurich]

This is a fascinating conclusion, because if correct it provides a much more straightforward assignment of blame to mankind, using what appears to be a more scientifically sound methodology (i.e. the energy balance).

III. The Blame Game -- Maybe Not Quite So Accurate

Unfortunately there are some significant issues with the paper which cause its conclusions to be brought into question.  

One notable is the issue is whether the scaled forcings are truly of "equal efficacy".  It's quite possible that different forcings could operate fundamentally differently, given where they put their heat (e.g. atmosphere v. ocean, etc.).  Problematically, the paper's authors fail to provide much explanation of why they think this approach is truly valid.

Secondly the study's methodology section states that another potentially problematic assumption was made:

It is assumed that the feedbacks are constant over time and the forcing uncertainty can largely be captured by a time-independent scaling factor.

This gets back to question 2 of the big questions previously raised -- "How far can the system go before reaching equilibrium?"

The model essentially is assuming that the Earth is going to sit here warming, without any significant cooling counter-effect being produced.  Geological record, as well as scientific common sense would cast doubt on this notion.  To give on example, past warming has altered sea currents, leading to a net cooling trend.

Finally, the paper admits that something strange is going on heat balance-wise with the ocean:

Ocean heat uptake for 3,000m depth is also larger, but the model is only constrained using data to 700m depth. Furthermore, uncertainties in ocean heat uptake are large and differences between various reconstructions are significant. The near constant ocean temperature over the past five years are not simulated by the model and its causes remain unclear.

For some reason the ocean temperatures have flatlined.  But the papers ignore this data, opting to only use the data that indicates nice, homogenous warming.

Observation comparisons
The energy balance results are compared with observational data, but the researchers questionably neglect comparisons for the last five years of ocean data.
[Image Source: ETH Zurich]

Thus, at the end of the day while this paper makes some progress in adopting a more sensible metric for quantifying warming (energy balance) and uses some impressive simulation techniques (e.g. the neural network trained to performing the BERN2.5D model Markov Chain Monte Carlo simulations), the uncertainty in its conclusions is likely understated.

Is mankind responsible for some amount of warming?  Certainly -- again, basic physics tells us this.  But is this warming permanent and linearly scaling with greenhouse gas emissions?  The answer there is far less certain.

It would be terrifically convenient to blindly accept the paper's conclusion and take it as dogma that mankind is responsible for 74 percent of warming.  Unfortunately taking a look at the paper's methodology from an analytical and scientific perspective reveals that doing so is likely folly.

The authors deserve praise for applying the energy balance and for providing explicit qualifications for some of the key assumptions and shortcomings of their work.  But the paper does not definitively pin what percent of warming mankind is to blame for. As The X Files states, "The truth is out there."

Hot summer day
[Image Source: Sustainable Blog]

It is important for politicians to bear this state of uncertainty in mind, when deciding on whether to adopt more costly means of "fighting" global warming by cutting greenhouse gas emissions.  Some methods like deploying nuclear energy or funding novel solar energy research represent lower cost, lower risk scenarios, in which mankind would still benefit in other ways, even if the warming threat proves overstated.  But when it comes to more extreme actions, such as adopting meat rationing or carbon credits (wealth redistribution) schemes, it seems a bit premature to resort to such expensive schemes given the lack of definitive understanding.

Source: ETH Zurich [PDF draft]



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RE: DT Bias
By Mint on 12/27/2011 2:14:17 PM , Rating: 2
quote:
But even relatively efficient A/C's (those with an SEER = 13) require 3.4 units of energy for each unit of thermal energy they remove.
You got that backwards. SEER 13 is roughly equal to a COP of 3.4, and that means it takes 1J of electrical energy to remove 3.4J of heat.

SEER is BTU/Wh. Ideally, that means those 3412 BTU needs 3412/13=262Wh=0.262kWh of energy by your A/C.

But your point still stands. Reduce an appliance's usage by 1kWh, and your bill goes down more in the summer (1.3kWh).


RE: DT Bias
By chemist1 on 12/27/2011 11:23:15 PM , Rating: 2
Ah, yes I did (invert the numbers) :). Thanks for the correction.


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