Climate Realism
Understanding Agreement & Disagreement inClimate Science
University of Warwick, 19 February 2018
Dr Benny Peiser
Director, Global Warming Policy Foundation (GWPF)

What is Climate Realism?

Acknowledging Knowledge and Ignorance There are many scientific agreements and disagreements in climatescience.

 This talk is not about who is right and who is wrong.
 The key questions of my talk: Which knowledge claims are reliable and trustworthy and which areless so?

 What do we really know about terrestrial climate change?
 Why do we accept certain scientific claims about climate change but

are doubtful about others?

What is Climate Realism?
What do we reliably know about terrestrial climatechange?
RECENT & PRESENT: more robust knowledge based on empirical observations andverifiable temperature measurements



PAST: ambiguous knowledge based on circumstantial evidence and estimates of
paleoclimate proxy data



FUTURE: Climate is a highly complex, chaotic and non-linear system; affected bynumerous known & unknown factors, dynamics and feedback loops, i.e. long-term futureclimate cannot be predicted reliably.

Empirical vs Theoretical Knowledge Empirical evidence - verifiable data and replicable methods.
 Reliable data and reliable estimates & predictions diminish the further one

moves back or forward in time.

 Modern warming trend (since ~1850) generally agreed because

observational data is fairly robust and based on verifiable measurements.

 Controversy increases as climate research moves further back in time andfurther into the future.

 Climate realism acknowledges significant difference between verifiableknowledge and hypothetical knowledge based on indirect evidence.

General Agreement
The Modern Warm Period

Met Office’s HadCRUT4 dataset starts in 1850 -- not enough temperature data before then to reliably constructglobal temperature.

http://www.ysbl.york.ac.uk/~cowtan/applets/trend/trend.html

General Agreement
Global mean temperature has risen by ~1°C since 1850



CO2 has increased in the atmosphere from approximately 0.030% to 0.040% (or 400 parts
per million, ppm) over the past 50 years.

CO2 is a greenhouse gas and the greenhouse effect is real.

Anthropogenic CO2 emissions have been rising steadily and have been contributing toglobal warming.

Since 1990 global temperatures have risen between 0.13 °C and 0.19 °C per decade,depending on which of the official data sets is used.

This warming trend is a third to two-thirds of the rate predicted by the IPCC in 1990 (0.3°Cper decade) and slower than most climate models forecast.

Human & Natural Causes of Climate Change

The IPCC Climate Change Consensus

"It is extremely likely that more thanhalf of the observed increase inglobal average surface
temperature from 1951 to 2010 wascaused by the anthropogenic
increase in greenhouse gas
concentrations and otheranthropogenic forcings together.“– IPCC 5th Assessment Report 2013
USGCRP, 2017: Climate Science Special Report: Fourth National Climate
Assessment, Fig 3.1

1. Disagreement
Quantifying Anthropogenic and Natural Climate
Change How do scientists estimate how much of the modern warming is due

to anthropogenic CO2 emissions and how much due to naturalvariability?
 Very difficult task due to lack of sufficiently reliable knowledge ofnatural variability over long periods of time (decades, centuries,millennia).
 If natural variability of past climate change is low, human factor inmodern warming is more significant.

 If natural variability is more significant, anthropogenic contribution toclimate change is smaller.

Estimating Natural VariabilityHow do scientists estimate how much of the modern warming is due to natural variability andhow much due to anthropogenic CO2 emissions?

IPCC, 2013, Fig SPM.5

What if natural variability of historical climatechange is much more pronounced?

R. Connolly, and M. Connolly (2014)

The problem is that all paleo-climatologicalreconstructions of historical climate change
rely on proxy-data that are inherently
vague, uncertain and often contradictory =GIGO problem

2. Disagreement
Positive vs Negative Feedbacks


Most climate models assume that
positive feedbacks in the climate
system far outweigh negativefeedbacks.

How do clouds react to changes intemperature?

Do they counter warming or amplify it?

How clouds affect global temperature
is one of the most contentious
problems in climate science.

Positive Feedbacks cause strong
global warmingWithout any feedbacks, a doubling of
CO2 would result only in 1°C globalwarming (IPCC 2007).

If water vapor and clouds respond to
this small rise in temperature by
amplifying the warming it would
constitute a positive feedback.

If they counter-balance the warming,
it would be a negative feedback.

3. Disagreement
Climate SensitivityThe equilibrium climate sensitivity refers to
the change in global temperature thatwould result from a doubling of
atmospheric CO2 concentration



Most climate models suggest that theequilibrium climate sensitivity is likely to bein the range 1.5°C to 4.5°C, with a bestestimate value of about 3°C.

From: Lewis and Crok (2014)

Climate Sensitivity
Models Vs Observations
Climate sensitivity estimates based on computermodel simulations suggest ~3°C per doubling ofCO2.



Empirical estimates of climate sensitivity fromobservational data such as temperature andocean heat records suggest much less (~2°C orless).

Climate realists trust observational studies more,claim that climate models display too muchsensitivity to CO2 & exaggerate future globalwarming.

source: Lewis and Crok (2014)

4. Disagreement
How warm was the Medieval WarmPeriod?
The Medieval Warm Period lasted from about 950 to 1250.

Because paleo-climate proxy records as well as their selection and interpretation
are highly contentious, there has been controversy about the magnitude andextent of the MWP.

The IPCC concludes MWP was not as warm as today, many sceptics claim MWP
was warmer or as warm than today.

Reality: We do not know with any degree of confidence whether MWP was global
or how warm it was.

We do not know what caused the MWP -- possible causes include increased solar
activity, decreased volcanic activity, and changes to ocean circulation.

The Medieval Warm Period
How the IPCC sees the MWP

Hockey stick chart from the 2001 IPCC Third Assessment Report,
showing Northern Hemisphere surface temperatures (departures fromthe 1961-1990 average, in degrees Celsius) of the past 1,000 years.

How climate sceptics see the MWP

R. Connolly, and M. Connolly (2014)

The Holocene Temperature
Conundrum
Paleo-climate proxy records suggest global
cooling during the Late Holocene, following
the peak warming of the Holocene ThermalMaximum (∼10 to 6 ka) until the rapidwarming of the modern warm period(Marcott et al., 2013)“This global cooling is puzzling because it is
opposite from the expected and simulated
global warming trend due to the retreating
ice sheets and rising atmosphericgreenhouse gases. “ – Liu et al. 2014

Liu et al. 2014

5. Disagreement
How Exceptional is the Modern WarmPeriod?
NOAA (2015) conclusion: “Overthe past two millennia, climate
warmed and cooled, but noprevious warming episodes
appear to have been as large
and abrupt as recent globalwarming.”

The Jury Is Out
Previous warm periods in China
“The reconstruction was developed using
proxy temperature data from fivelocations across China, and an observed
temperature dataset provided by
Chinese Meteorological Administration
covering the decades from the 1870s tothe 1990s.”

“Relative to the 1870s–1990s climatology,
our two reconstructions both show threewarm intervals during the 270s–390s,1080s–1210s, and after the 1920s;
temperatures in the 260s–400s, 560s–730s
and 970s–1250s were comparable withthose of the Present Warm Period.” --Geet al. 2013

Ge et al. 2013

Conclusion
The Science Isn’t Settled Human and natural factors are contributing to global warming

It is currently impossible to reliably quantify their specific contributions.
Feedbacks in a highly complex system such as the climate of planet Earth are extremely
difficult to quantify.
The biggest uncertainty about future climate change is due to a manifest lack ofunderstanding of historical natural variability and the climate’s feedback mechanisms.
“The climate system is particularly challenging since it is known that components in thesystem are inherently chaotic; there are feedbacks that could potentially switch sign, andthere are central processes that affect the system in a complicated, non-linear manner.
These complex, chaotic, non-linear dynamics are an inherent aspect of the climatesystem.” – IPCC 1990
“The climate system is a coupled non-linear chaotic system, and therefore the longterm prediction of future climate states is not possible.” – IPCC 1990