Global Climate Change: What do we know & What don’t we know?*

Klaus Wolter

University of Colorado at Boulder, klaus.wolter@noaa.gov

Kudos to Susan Solomon (most unmarked slides)

(Co-Chair of WG 1, IPCC 4; a.k.a. “The Horse’s Mouth”)

• Background on IPCC & Global Change modeling

• Observed climate changes & how well they are modeled

• Projections through the 21st century

• Closing thoughts

* With two nods to our former Secretary of Defense (also: “you have to analyze climate change with the data you have got…”)

Structure of the IPCC

Socio-economics, policy options, discount rates, emission scenarios,…..

Flowering dates, corals, coastal zone erosion,….

WG1 - Climate Change:The Physical

Science Basis

WG2 - Climate Change: Impacts and Adaptation

WG3: Mitigation

Three different working groups with quite distinct scientific purviews and required expertise. IPCC assesses research but it doesn’t do research. WG1 report is at ipcc-wg1.ucar.edu.

The IPCC Sequence

IPCC (1990) Broad overview of climate change science, discussion of uncertainties and evidence for warming.

IPCC (1995) “The balance of evidence suggests a discernible human influence on global climate.”

IPCC (2001) “Most of the warming of the past 50 years is likely (>66%) to be attributable to human activities.”

IPCC (2007) “Warming is unequivocal, and most of the warming of the past 50 years is very likely (90%) due to increases in greenhouse gases.”

Intergovernmental Panel on Climate Change, UN-approved since 1988, 75% of 2007 WG-1 authors did not work on last assessment (2001,Third Assessment Report).

Latest info on GHG

There is no debate whether greenhouse gases are rising - the evidence is clear-cut that their (carbon dioxide, methane+) levels have been rising to (& at) unprecedented levels (rates) longer than homo sapiens has existed (at least 600K, actually), and as best as I can tell there is little debate as to whether this increase is human-caused.

IPCC, 2007: SPM 2

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Last Ice Age

Humans are ‘forcing’ the earth system in a new way. CO2 increases due to fossil fuel burning are the dominant cause of global warming. CO2 has not been this high in more than half a million years.

Last interglacial

Ice ages are not random. They are 'forced' (by earth’s orbital clock…. changes in the sunlight received).

350

300

250

200

Carb

on

Dio

xid

e A

mou

nt

(pp

mv)

600 500 400 300 200 100 0

Thousands of Years Before Present

[Adapted from Figure 6.3, ©IPCC 2007: WG1-AR4]

Better and longer satellite data about the Sun

Improved assessment: a) no significant observed trend in solar irradiance since 1978

using high quality inter-calibrated data; b) spectral information c) solar magnetic flux model rather than proxy data; d) re-evaluation of variations in Sun-like stars.

Solar irradiance forcing much smaller than GHG, and then TAR estimates.

Observations reveal the presence and provide quantitative aspects. Aerosol transport-forcing models better tested and constrained. Much improved estimate of the Aerosol Direct Radiative Forcing.

January to March, 2001

New information about aerosols, and major modelling improvements

Human and Natural Drivers of Climate Change

1.6 W m-2 warms like 1.6 xmas tree lights over every m2 on Earth.

Carbon dioxide is causing the bulk of the forcing, and it lives a long time in our atmosphere so every year of emission means commitments to climate change for future generations.

IPCC, 2007: SPM 2

Figure 10.1

IPCC, 2007: 10.1

Figure 3.1

IPCC, 2007: 3.1 - Observed temperature changes since 1850 (different QC/averages)

Observed climate changes & how well they are modeled

Correlativity of U.S. climate stations (DJF temps 1979-03)

Correlativity tends to be highest in the winter. Iowa winter temperatures could be monitored by just a few (one?) stations, while Colorado has many more ‘local flavors’. (Wolter and Allured; http://www.cdc.noaa.gov/people/klaus.wolter/ClimateDivisions)

Figure 3.2

IPCC, 2007: 3.2 - land data

IPCC, 2007: 3.9 - °C per century on left/per decade on right (and below); grey = insufficient data

IPCC, 2007: FAQ 3.1 - MSU-based trends for last 25 years (better match than in TAR)

Figure 3.10

IPCC, 2007: 3.10 - 18 years of data (≥2 months per season) requred; sensitive to end points!

Figure 3.12

IPCC, 2007: 3.12 - a lot more “enthusiasm” about wetting trend in/after TAR!

Figure 3.13

IPCC, 2007: 3.13 - note contrast between two periods in Western U.S. and Sahel!

Figure 4.1

IPCC, 2007: 4.1 - Components of cryosphere & their time scales

Figure 4.2

IPCC, 2007: 4.2 (March-August dropping; September-February NOT - WHY?)

Figure 4.8

IPCC, 2007: 4.8 - NH dropping (spring& summer!); SH not - WHY?

Figure 4.15

IPCC, 2007: 4.15 - specific vs. total glacier mass balance (regional strength of climate change vs. contribution to sea level rise)

Figure 4.23

IPCC, 2007: 4.23 - Summary slide

Attribution• Asks whether observed changes

are consistent with expected responses to forcings inconsistent with alternative

explanations

• Most of the observed increase in globally averaged temperatures since the 1970s is very likely (>90%) due to the observed increase in anthropogenic greenhouse gas concentrations

TS-23

Anthro+ Nat forcing

Understanding and Attributing Climate Change

Anthropogenic effect on warming

averaged over each continent except

Antarctica is likely [IPCC, 2007]

Observed

Expected for all forcings

Natural forcing only

Understanding and Attributing Climate Change

Anthropogenic effect on warming

averaged over each continent except

Antarctica is likely [IPCC, 2007]

Observed

Expected for all forcings

Natural forcing only

Attribution studies

• Separate time-space patterns of response.

• Solar response has very different behavior to GHG, especially with altitude. The upper atmosphere would be expected to be much warmer than it is if solar irradiance were the cause of current surface climate change.

Solar

“All” forcings

Figure 3.27

IPCC, 2007: 3.27 - SLP/Tsfc/Precip vs. SOI - step change around ‘76?!

Figure 3.33

IPCC, 2007: 3.33 - AMO: main driver for Atlantic hurricane trends?!

IPCC, 2007: 3.38 (10th percentile for 1901-50 (black), 51-78 (blue), and 79-03 (red)

Figure 3.39

IPCC, 2007: 3.39 - contribution from very wet days (95th percentile) to annual precip

Figure 3.40

IPCC, 2007: 3.40 - Accumulated Cyclone Energy (ACE); still much debate on how much of the recent increase in North Atlantic is related to natural climate variability (AMO) vs. anthropogenic; there are also quite a few (poorly resolved) inhomogeneities in record (Landsea, 2007; pers. comm.)

Calculus of extremes

Standard deviation

1 in 40 yr high range

The distribution of weather events around the climatic average often follows a ‘bell-shaped’ curve.

Climate change can involve change in the average, or the spread around the average (standard deviation), or both.

A shift in temperature distribution has much larger relative effect in the tails than near the mean. For instance, huge JJA’03 anomaly in Europe (>5 sigma) is more consistent with higher sigma than with higher mean.

A shift of 1 standard deviation makes a 1 in 40 yr event into a 1 in 6 yr event

Projections through the 21st century

Figure 10.2

IPCC, 2007: 10.2

Committed Warming is Coming

• For the next two decades a warming of about 0.2°C per decade is projected for a range of emission scenarios.

• Even if the concentrations of all greenhouse gases and aerosols were to be kept constant at year 2000 levels, a further warming of about 0.1°C per decade would be expected.

The Longer TermWarming will increase if GHGs increase. If GHGs were kept fixed at current levels, a committed 0.6°C of further warming would be expected by 2100. Higher emissions translate into more warming.

1.8oC = 3.2oF

2.8oC = 5.0oF

3.4oC = 6.1oF850

600

4000.6oC = 1.0oF

Projections of Future Changes in Climate: Probabilistic Information for the First Time

Changes over next few decades already ‘locked in’ (to the extent that the models can provide the answer), big differences by end of century

Seasonal moisture changes as projected for DJF and JJA

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Wetter in winter & drier in the summer, if you believe the model average…

Bottomline on regional climates (&trends)

VAR = GHG + Decadal + ENSO + Regional + Local + Noise

(not everything is greenhouse-related, nor will it ever be & don’t underestimate the “noise”=our (in-) ability to measure correctly)

We do NOT fully understand all of the above components of the climate system, leaving room for surprises!

This is my opinion, not the IPCC’s!

Figure 10.14

IPCC, 2007: 10.14 - Sea ice projections

Figure 10.16

IPCC, 2007: 10.16 - are we going to get more/stronger El Niños???

Figure 10.18

Figure 10.32

IPCC, 2007: 10.32 - sea leve change for end of 21st century relative to expected global average change

Figure 10.38

Figure 11.12

IPCC, 2007: 11.12 - problem with regional ‘down-scaled’ scenarios: models that are good in present are not guaranteed to stay good; high fidelity in temp reconstructions/ projections does NOT translate into good precip reconstructions/projections

Box 11.1, Figure 2

Some Pressing Needs for a WG1 AR5…. (Susan Solomon)

• Forcing: How well do we understand past history of emissions and forcing? Future? Aerosol effects on precip? Role of soot? Land use? Stabilization of GHG (if governments so decide) is linked to understanding of forcing/feedback relationships (esp. carbon).

• Beyond global warming: The AR5 will likely advance the understanding of hurricanes, drought, heat waves, other extremes, precipitation patterns, ocean circulation, and other ‘earth system’ variables, for observations, attribution, and projections. Much more regional information will likely be sought, and as in the AR4, this should follow on the basis of physics (not catalogues). Central to understanding the adaptation/mitigation challenges.

• Near-term: Already committed to more warming (next few decades), with choices about emissions affecting the longer term more and more…commitments to SLR? Drought? Heat waves? Hurricanes?

• Long term: SLR changes the face of the planet. The next assessment will likely advance understanding of ice sheets and SLR.