- Interpreting the Warming

Huang-Hsiung Hsu

Research Center for Environmental Changes Laboratory for Climate Change Study

台灣欒樹

Koelreuteria formosana

鳳凰樹 Poinciana

Interpreting the Warming - What did IPCC AR5 say and why

ASIAA/CCMS/IAMS/LeCosPA/NTU-Phys Joint Colloquia 1 October 2013

http://www.scidacreview.org/0902/html/esg.html It is extremely likely that

human influence has been the dominant cause of the

observed warming since the mid-20th century.

Evidence and confidence on the human influence on the recent warming have been growing from IPCC FAR (SAR, AR3, AR4) to

the Fifth Assessment Report (AR5).

IPCC: Intergovernmental Panel for Climate Change AR5 (released on 27 and 30 September 2013):

Fifth IPCC summary of literature published before March 2013 (209 authors, >9200 papers cited, 54677 comments, …)

Earth climate has been warming.

Earth climate has been warming.

Earth climate has been warming.

Also in sensitive variables that feedback positively to temperature change

• ice-albedo feedback

• heat release from ice- free ocean surface

• shorter winter, longer summer

• CO2, CH4 release from ice-free water and thawing land surface

• more human activities:

shipping, mining, …

• …

Better variables

preserving warming signal in ocean

- monotonically warming trend

- huge heat bank

Better variables

preserving warming signal in ocean

- monotonically warming trend

- huge heat bank

Greenhouse gase concentration has been and continue rising.

CO

(CH

) concentration increase since the Industrial Revolution has exceeded far beyond the natural variation in past 800,000

years. Is this responsible for recent warming?

Credit: Jeremy Shakun/Harvard University

Greenhouse gase concentration has been and continue rising.

CO

(CH

) concentration increase since the Industrial Revolution has exceeded far beyond the natural variation in past 800,000

years. Is this responsible for recent warming?

Credit: Jeremy Shakun/Harvard University

Back-to-the-past (future) virtual reality simulation

Climate model: atmos., ocean, land, vegetation, biogeochemical cycle, ……

greenhouse gases

aerosol

volcanic ash

solar radiation

Anthropogenic or natural effect?

land use change

Natural vs. anthropogenic forcings

It is extremely likely that human influence has been the dominant cause of the observed warming since the mid- 20th century.

Natural vs. anthropogenic forcings

How to project the future?

Carbon Dioxide Emission Scenario (RCP, AR5)

IPCC AR5 Mitigation Pathway

421 ppm

Stabilization Pathway 538 ppm

Stabilization Pathway 670 ppm

High emission Pathway 936 ppm

Present: 395 ppm

IPCC AR5

1.0ºC 3.7ºC

0.3ºC 4.8ºC

0.4m 0.63m

0.26m 0.82m

Natural variation？Anthropogenic effect？

How good is our chance to avoid 2ºC warming?

Tipping Point?

Cumulative emissions of CO2 largely determine global mean surface warming by the late 21^st century and beyond.

Cumulative emissions of CO2 largely determine global mean surface warming by the late 21^st century and beyond.

“Continued emissions of greenhouse gases will cause further warming and changes in all components of the climate system.

Limiting climate change will require substantial and sustained reductions of greenhouse gas emissions.” - IPCC AR5

Different scenarios yield different projection. We have a choice.

End of 21^st Century, Asia

RCP8.5: stronger extreme rainfall, no-rain day frequency increase

RCP2.6: weaker extreme rainfall, no-rain day frequency decrease

Seasonal range [DJF-JJA] in 99% intensity (mm/day).

-40 40

-15 15

Seasonal range [DJF-JJA] in frequency and no rain days (days/season)

RCP2.6 RCP4.5 RCP6 RCP8.5

Nicolas Freychet

• Global warming is a projection, not prediction, based on scenarios. If scenarios do not occur, the projection may not happen.

• Although projection tools are not perfect, they are the best tools in the human history.

• Global warming projection contains uncertainty. It is not a pure scientific issue. It is a matter of choice and a risk assessment and management issue.

• Key question: What should we do , in view of future warning, to minimize the potential

impact of future global warming?

The Joint Program on the Science and Policy of Global Change, MIT http://globalchange.mit.edu/index.html

Future is in our hands.

Some counter arguments

• Earth climate had fluctuated in a much larger amplitude.

2-3ºC warming is nothing in the Earth history.

AR4

True. But it is a matter of time scale.

• Warming has been slowing down since 1998 and may stop.

But ocean heat content (sea level) continues increasing.

Global mean temperature (1880~2005)

氣象局陳雲蘭提供 Observed change

＝centennial＋interdecadal＋interannual

Centennial trend Inter-

decadal

Inter- annual

And, again, it is matter of time scale.

• Arctic sea ice extent increased by 60% in 2013. Warming has stopped; instead, global cooling may start.

• Matter of time scale, again!

2013 was the year with 6^th smallest sea ice extent and still lying along the downward trend

• How about solar cycle?

Changes in solar irradiance have been small (0.05Wm

)

compared to other radiative forcings (GHG ~ 3Wm

Feulner, G., and S. Rahmstorf (2010), On the effect of a new grand minimum of solar activity on the future

climate on Earth, Geophys. Res. Lett., 37, L05707, doi:10.1029/2010GL042710.

Temperature decrease due to “future grand minimum of solar activity” is much smaller than the warming

expected from anthropogenic greenhouse gas emissions by the end of the century.

What will happen if “Maunder Minimum”

occur again in the future?

Geoengineering: Hopes for offsetting future warming?

Solar Radiation Management (SRM)

Carbon Dioxide Removal (CDR)

AR5 Assessment

• CDR methods have biogeochemical and technological limitations to their potential on a global scale. There is insufficient knowledge to quantify how much CO2 emissions could be partially offset by CDR on a century timescale.

• Modelling indicates that SRM methods, if realizable, have the potential to substantially offset a global temperature rise, but they would also modify the global water cycle, and would not reduce ocean acidification.

• If SRM were terminated for any reason, there is high confidence that global surface temperatures would rise very rapidly to values consistent with the greenhouse gas forcing.

• CDR and SRM methods carry side effects and long-term

consequences on a global scale.