Three most common questions about climate change:

1. Is it real?

2. Is it caused by humans?

3. What should we do?

“On the influence of carbonic acid in the air on the temperature of the ground” - Svante Arrhenius, 1896

This is not new science!

FINDING #1: The “Greenhouse effect”

96 % CO2 460 °C

~0.3% CO2 15 °C

FINDING #2: Greenhouse gas levels in the atmosphere have

increased because of human activity

CO2 levels (ppmv):Since the ice age 280Mid-1800s 280Mid-1950s 315Today 370

End of ice age280Mid-1800s 280Mid-1950s 315Today 370

Source: NOAA ESRL

““Proof”: Decreasing fraction of Proof”: Decreasing fraction of 1313C in atmospheric C in atmospheric COCO22

Total emissions

Atmospheric increase

Source: IPCC

Emissions do not necessarily stay in the atmosphere!

800 billion tons of carbon (STOCK or

POOL)

9 billion tons of carbon / year (FLOW or FLUX)

2billion tons/yr

3billion tons/yr

LAND OCEANS

Today: 4 billion tons of carbon or 15 billion tons of CO2

are being added to the atmosphere each year = almost 2 parts per million (ppm)

Lesson #1

FINDING THREE: Temperature has increased

Why are there so many datasets?

Observations of global rise in sea level

1.8 ± 0.5 mm per year since 1960

SHORTER LAKE ICE “SEASON”

PLANTS FLOWERING EARLIER

PLANTS AND ANIMALS MIGRATING NORTH [AND HIGHER]

CORALS “BLEACHING”

OCEANS WARMING AND RISING

LESS SNOW COVER

LESS ARCTIC ICE

WARMER RIVERS AND LAKES

WARMWATER FISH MOVING NORTHBIRDS MIGRATING

EARLIER

MORE FIRESDISEASE AND PEST OUTBREAKS

THREE FINDINGS

Greenhouse gases warm the Earth’s atmosphere

Greenhouse gas levels in the atmosphere have increased because of human activity

The climate has warmed

Attribution of climate change

We have done “detection”. Now the hypothesis is:

The global climate has warmed since the Industrial Revolution due to the emission of greenhouse gases by human activity.

H0: Not due to human activity (e.g. other factors at play)

Ha: Due to human activity

We need to do the math! What is the magnitude of the enhanced greenhouse effect? What other factors have influenced the climate since the Industrial Revolution (e.g. the sun, internal variability)?

Probabilistic or Bayesian “truths”

It is extremely unlikely (<5%) that the global pattern of warming observed during the past half century can be explained without external forcing.

It is very likely that anthropogenic greenhouse gas increases caused most of the observed increase in global average temperatures since the mid-20th century…

We expect certainty from science when it delivers probabilities

1.Warming in the troposphere, cooling in the stratosphere

2.More warming at night-time

3.Less radiation escaping to space at wavelengths of greenhouse gases

(Winters warming faster than summers, more warming in the Northern Hemisphere, more warming at high latitudes, etc.)

Are models the only “smoking gun”?

IPCC 2007

Temperature change by atmospheric layer

IPCC 2007

Warming faster at night

Starting in 1996, the Japanese ADEOS satellite, recorded global observations of the spectrum of outgoing longwave radiation.

Some 27 years earlier, NASA had a similar instrument on the Nimbus 4 spacecraft, between 1970 and 1971.

Absorption of outgoing longwave radiation

FINDING FOUR: The fingerprints

Multiple lines of evidence related to the pattern of warming and the magnitude of “natural forcings” suggests humans are the primary cause

Neither the future, nor the past, can be known with 100% certainty.

We must evaluate predictions, whether from models or from individuals, probabilistically.

Hypothesis testing

Question: Is the climate changing? (“detection”)

Hypothesis: The global climate has warmed since the Industrial Revolution

Null hypothesis H0: No trend in climate

Alternative hypothesis Ha: Trend in climate

As opposed to weather (“what you get”), climate (“what you expect”) is a statistical property. The answer to our question requires an evaluation of whether the signal (the trend) is statistically distinct from the noise.

A signal to noise problem

Source: NCDC

A signal to noise problem

Source: NCDC

A signal to noise problem

Source: NCDC

A signal to noise problem

Source: NCDC

Observed global mean temperature is on the lower end of the range over the past five years.

Should models be able to simulate the climate of a particular year, or decade?

We ask every time there’s a heat wave or extreme event:

Is this [BLANK] caused by global warming?

Cover of Bloomberg Businessweek, owned by the Mayor of New York City, after Hurricane Sandy struck New York

There are several ways, statistically-speaking, in which the climate might change (in response to some external or internal forcing – these examples are not specifically about greenhouse gases!)

Source: IPCC

In these cases, a temperature extreme is more likely to occur due to climate change. However, it could also have occurred before the climate change.

So, a heat wave may represent of the type of weather more likely to occur due to global warming, but is not necessarily caused by global warming.

But what if the event does not have a precedent (in the available data***)?

March 2012 (NASA data)

Probability and Climate Changehttp://www.nasa.gov/topics/earth/features/warming-links.html

Did Barry Bonds hit home runs before he took steroids?

Could Lance Armstrong have one the Tour de France without doping?

“The ultimate objective of this Convention and any related legal instruments that the Conference of the Parties may adopt is to achieve”

UN Framework Convention on Climate Change (Article II)

“stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.”

Source: IPCC, Hansen et al., 2008; 350.org

“350 (ppm) is the number that leading scientists say is the safe upper limit for carbon dioxide...it's the number humanity needs to get back to as soon as possible to avoid runaway climate change.”

From Cancun Accord, signed at 17th COP to UNFCCC:

“… deep cuts in global greenhouse gas

emissions are required according to science…

with a view to reducing global greenhouse gas

emissions so as to hold the increase in global

average temperature below 2°C above pre-

industrial levels… action to meet this long-term

goal, consistent with science and on the basis

of equity… strengthening the long-term global

goal on the basis of the best available scientific

knowledge, including in relation to a global

average temperature rise of 1.5°C”

Source: UNFCCC

“To protect at least 50% of the coral reef cells, global meantemperature change would have to be limited to 1.2 C (1.1 – 1.4 C)”

Is this a scientific question?

Science can provide objective judgments based on experimentation, evaluation of data and hypothesis testing.

The choice to take an action – whether set a pollution standard or to determine an “acceptable” level of greenhouse gases – is a normative judgment. It depends on the values of the individual.

This judgment may take science into consideration, as well as culture, economics, and the “zero-sum game” nature of decision making.

“Pathways” to +2 deg C

“Super wicked problems” are wicked problems with the following additional characteristics:

•Time available to solve the problem is running out.

•There is no central authority to impose a solution.

•Those in the best position to solve the problem are also causing it

Lazarus, R.J. 2010. Super wicked problems and climate change: restraining the present to liberate the future. Cornell Law Review 94: 1153-1234.

Climate change: a “Super Wicked Problem”