Michael MacCracken, Ph.D. – Chief Scientist, The Climate Institute
US-TAG Meeting, Washington DC
August 4, 2014
IPCC Assessment Reports have been a key reference in better
understanding the science.
Intergovernmental Panel on Climate Change (IPCC) –
organized in 1988 by the United Nations and World
Meteorological Organization to assess and summarize
international scientific and expert understanding.
IPCC’s assessment reports – Compiled by three
working groups, covering the physical science of
climate change (WG 1), its impacts (WG 2), and
options for addressing it (WG 3). IPCC does not
conduct its own research, nor does it monitor climate
related data.
Five assessment reports published since 1990, each
unanimously accepted by its 190+ nation membership.
Fifth Assessment Report (AR5) – WG 1 report
approved in September 2013. WG 2 and WG 3 reports
approved in April 2014. A Synthesis Report is now in
Global warming and sea ice retreat are occurring rapidly, all
convincingly attributed to human activities.
While projected increases in global average temperature vary by emissions
scenario, all projections for uncontrolled emissions lead to temperature
increases that would cause much more change in the future.
Increase in global temperature since 1750 (°C)
High scenario
Medium scenario
Low scenario
All emissions scenarios show
global average temperature
increasing to levels greater than
2°C above pre-industrial values.
Note: Temperature scale used by IPCC has
been adjusted so the baseline is the average
near the start of the Industrial Revolution.
All projections indicate that warming will be greatest at
high latitudes, over land, during winter, and at night,
except where land dries out in spring and summer.
Most land areas are
projected to warm by
~3-5oC by 2100; the
Arctic warms by
much more.
Over oceans and
low latitudes,
evaporation buffers
While fossil fuels significantly benefit society, the impacts of climate
change on several important sectors will become very significant.
Health Impacts
Changes in CO2 and climate will cause:
Warming and extreme
Precipitation, floods and
Sea level rise and melting
glaciers and ice
Ocean acidification
Adapted originally from EPA
Societal Impacts
• Indigenous peoples and those in
developing nations
• Exacerbated impacts on the poor and
those in urban areas
• Dramatically different situation for
future generations
Weather-related mortality/heat stress
Infectious diseases
Air quality-induced respiratory effects
Agriculture Impacts
Crop yields and commodity prices
Irrigation demands
Pests and weed
Forest Impacts
Change in forest composition
Shift geographic range of forests
Forest health and productivity
Water Resource Impacts
Changes in water supply and timing
Water quality
Increased competition for water
Coastal Area Impacts
Erosion of beaches
Inundation of coastal wetlands
Costs to defend coastal communities
Ecosystem Impacts
Shifts in ecological zones
Loss of habitat and species
Coral reefs threatened
Given such large projected changes, the
UN Framework Convention on Climate Change
(UNFCCC) was negotiated and approved in 1992.
UNFCC Objective 2 called for:
Stabilization of greenhouse gas (GHG) concentrations in the atmosphere at a
level that would prevent dangerous anthropogenic interference with the
climate system.
Such a level should be achieved within a time-frame sufficient
to allow ecosystems to adapt naturally to climate change,
to ensure that food production is not threatened, and
to enable economic development to proceed in a sustainable manner.
Carbon dioxide equivalent (CO 2e) is the
basic metric used to summarize the relative
contribution of various GHGs to climate change.
CO2e is calculated using a “global warming potential” (GWP)– a factor representing
the potency (in terms of “radiative forcing”) of each GHG relative to CO 2 over a set
time period.
 By convention, this relative potency is typically assessed over a 100-year time
period (GWP-100) .
 The 100-year period provides a means to evaluate the relative contribution of
emissions for each chemical species to long-term “peak warming,” which is
projected to range up to 6°C.
 The metric has been limited to six types of GHGs defined in the Kyoto Protocol.
 This focus on the 100-year time period and the Kyoto-listed GHGs has obscured
the importance of the near-term warming influence of methane and short-lived
climate forcers (SLCFs) – GHGs and aerosols.
Scientific studies indicating greatly increased likelihood for “dangerous
anthropogenic interference with the climate system” above 2°C led international
leaders to reset their goal to limit warming to 2°C, projected to occur ~2050
Increase in global temperature since 1750 (°C)
High scenario
Medium scenario
Low scenario
Can Adapt?
Note: Temperature scale adjusted so the
baseline is the average near the start of the
Industrial Revolution
+2 °
+4 °
Three global mean temperature (GMT) anomaly levels have
been suggested as critical upper limits by international
negotiators, each a different number of decades ahead.
Copenhagen Accord (2009)
114 Parties under the UNFCCC agreed
that the potential consequences of a GMT
increase of more than about 2°C above
the pre-industrial baseline are likely to be
All major nations, including the United
States, were parties to the agreement.
Negotiations are underway to agree on an
international approach in 2015.
1.5°C was initially considered as the
internationally agreed-upon threshold.
Given the changes already evident at
present warming levels (~ 0.8°C), the
1.5°C threshold will be reconsidered as
the appropriate goal in 2015.
IPCC AR5 makes clear that CO 2, methane, black carbon, and
HCFCs have been the main contributors to global warming.
Analysis of the relative contributions to radiative forcing
out to 2100 show that methane and tropospheric ozone
forcing equal 21 st-Century carbon dioxide forcing
Radiative Forcing (W/m2)
Projected reduction in
sulfate aerosols would
also cause a positive
N2 O
Forcing from 21st century CO2
emissions only
Forcing from 20th century GHG emissions
When the forcing due to black carbon is added in, it is
clear that forcing by short-lived species is well more than
half of total forcing from 21 st-Century emissions
Radiative Forcing (W/m2)
N2 O
Forcing from 21st century CO2
emissions only
Forcing from 20th century GHG emissions
Given that climate models project that we are likely to reach
2°C warming by ~ 2050, it is time to reconsider the GWP100.
Reliance on CO 2e based on the 100year GWP masks potential
approaches for addressing near-term
climate change out to 2050.
For policy development, planning, and
implementation, updates metrics are
needed that reflect the relative
importance of:
 limiting emissions of each of the
various species,
 in various regions
 when taking actions to meet
specific policy goals over specific
periods in the future.
The 2011 UNEP/WMO assessment on
tropospheric ozone and black carbon, for
example, made it clear that focusing on
the Kyoto GHGs alone is not sufficient.
• BC emissions: cut 80% by 2030
• CH4 emissions: cut 25% instead of
allow 25% increase by 2030
2.0 oC threshold
1.5 oC threshold
Shindell et al. 2012, From UNEP/WMO: “Integrated
Assessment of Black Carbon and Tropospheric Ozone”
The Climate and Clean Air Coalition to Reduce Short-Lived
Climate Pollutants (CCAC) Working Group convened
in Paris, France, from 16-17 July 2014.
CCAC has approved 10 initiatives, including seven
sectoral initiatives:
1. Accelerating methane and black carbon
reductions from oil and natural gas production
2. Addressing SLCFs from agriculture
3. Mitigating SLCFs and other pollutants from brick
4. Mitigating SLCFs from municipal solid waste
More than 90 participants attended, representing state
and non-state partners of the CCAC, its Scientific
Advisory Panel (SAP), the CCAC Secretariat and
5. Promoting HFC alternative technology and
6. Reducing black carbon emissions from heavyduty diesel vehicles and engines
7. Reducing SLCFs from household cooking and
domestic heating
With the impacts of warming occurring more rapidly than
previously projected, there is a clear need to identify the most
effective policy steps to support the setting of priorities.
Climate metrics need to account for:
• All climate forcers, including longlived species (e.g., CO 2, N2O, etc.)
and short-lived species (e.g.,
methane, black carbon, sulfate
and other aerosols, tropospheric
ozone, etc.)
PHOTO: Natural Resources Canada – Oxidation of
organic carbon into carbon dioxide and black carbon.
Black carbon is now recognized as the 2nd major
contributor to climate change, after carbon dioxide.
• All types of sources (e.g., fossil
fuel, biospheric, mixed, steady,
intermittent, etc.)
The metrics must be capable of reflecting the range of impacts
that are occurring in different regions and at different rates.
The metrics should support:
• Prioritization of steps that can be taken
to meet goals for different time horizons
(e.g., 2035, 2050, 2100)
• Prioritization of actions to limit specific
types of emissions in order to address a
wide range of impacts, such as:
 Overall amount of global warming
PHOTO: Geophysical Research Letters. Arctic
sea ice volume declined 36 % in the autumn and
9 % in the winter over the last decade since 2003.
 Amplified warming occurring in high
latitudes (and other regions)
 Ocean acidification
For example, in the Arctic, amplified warming is caused by a
combination of global warming by long-lived GHGs and further
regional amplification from emissions of short-lived species in
the region and beyond.
• The metrics should account not only for
emissions by type, but also for their potential
influence on regional “hot spots” such as the
Arctic, including seasonal variations in impacts.
• For example, black carbon would be expected
to have greater impacts when emissions occur
near regions of snow and ice, and in the spring
and summer when the Sun is up.
Temperatures in the Arctic have risen far
faster than the rest of the planet.
• Thus, while the amount of emission can effect
the intensity of the impact, so can location and
timing—and the impacts are what we are trying
to reduce or limit.
Moving to metrics that focus on impacts resulting from emissions
or activities rather than merely on their amounts should provide
greater insight into the most effective ways to take action.
• Upgraded climate accounting metrics, coupled
with sustainability standards, can help
governments and the public and private sectors
better prioritize actions to limit climate change.
• With future international harmonization through
ISO, the updated metrics have the potential to
facilitate both national and international progress
in limiting climate change, sea level rise, and
ocean acidification.
Strong Warning from IPCC AR 5 Regarding GWP-100
+2 °
+1.5 °
Heeding this warning, the
updated metrics should
encompass near-term
temperature thresholds,
including 1.5°C (2035) and
2.0°C (2050), as well as
4.0°C (2100).
+4 °
“The uncertainty in the GWP increases with time horizon, and for the
100-year GWP of WMGHGs the uncertainty can be as large as 40%.
Several studies also point out that this metric is not well suited for
policies with a maximum temperature target.” …IPCC AR5, TS3.8
Important assessments and well-reviewed scientific papers have
been used in the development of the updated metrics.
• Intergovernmental Panel on Climate Change (IPCC) Assessment Reports
• Arctic Monitoring and Assessment Programme (AMAP)
• National Aeronautics and Space Administration (NASA) analyses
• National Oceanic and Atmospheric Administration (NOAA) summary reports
on the climate
In addition, the updated metrics reflect:
• The findings of experts at the Scripps Institute of Oceanography, University of
Illinois, University of Michigan, and Stanford University related to black carbon
• Review comments from leading scientists in the field
Having the appropriate metrics is important because of the
many uses to which they are put.
 Inventories of activities and their coupling to impacts
 Indications of trends (and communication of trends to the public)
 Determining the coupling to the timing of emissions
 Policy development (setting goals and measuring progress)
 Setting financial incentives to actions (application across species)
 Implementation of Executive Order 13514 and Scope 3
 Valuing of emissions in trading or offset schemes
 International treaty negotiations (country-to-country comparisons)
 Private sector environmental initiatives
Examples of updated data and understanding
regarding short-lived climate forcers
Black Carbon
• IPCC estimates that black carbon forcing is 0.6 W/m 2. However, it aggregates
black carbon from biomass burning with cooling from organic carbon,
assuming the net result is zero. Estimates of forcing are not based on
observations of black carbon concentrations in the atmosphere, but instead
are based upon emissions data, which can be inaccurate for the main sources
of black carbon.
• Observational data from a consensus of leading experts has found that black
carbon contributes +1.1 W/m 2 to total global forcing.
Examples of updated data and understanding
regarding short-lived climate forcers
Cooling Aerosols
• Negative forcing from aerosols is uncertain, because of the significance of cloud
effects (which are difficult to sum up around the Earth), and the substantial
changes in emissions levels of aerosols over the past several decades.
• IPCC aggregates the forcing of all aerosols, including black carbon, to assess a
net ERF (effective radiative forcing) of -0.9 W/m2 that factors the indirect negative
forcing from cloud formation. However, this assignment has a low confidence
level and depends upon assumptions about the radiative forcing of black carbon.
• If black carbon forcing derived from observational data is correct, then the amount
of negative (cooling) forcing required to explain the current GMT anomaly leads to
an estimate of approximately -2.1 W/m2 for atmospheric coolants.
“Representative Concentration Pathways” (RCPs)
are used to define scenarios.
The IPCC has projected how the Global Mean Temperature (GMT) anomaly will evolve
over time, as radiative forcing changes over the 21st century. Four scenarios (“RCPs”)
are defined based upon the forcing level in 2100.
Only RCP 2.6 has the potential to stabilize the GMT anomaly at +1.5 to 2°C above
pre-industrial temperatures. In all other scenarios, +2.0°C will be passed by midcentury.
Source: IPCC AR5 Summary for Policymakers, Figure SPM.1.

Updating Climate Metrics: The Need for Consistency with