What is climate change?Tim Lenton

University of Exeter

Outline

A (geo)physiological view of the Earth

The diagnosis: Dangerous tipping points ahead

What to do about it? Medicate or tackle the underlying causes?

Atmospheric compositions

10-1

10-2

10-3

10-4

10-5

10-6

10-7 0.0000001

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

Earth Mars Venus

Mix

ing

Rat

io

HydrogenMethaneNitrogenNitrous OxideCarbon MonoxideCarbon DioxideOxygen

T. M. Lenton et al. (Working Group 1) 91st Dahlem Workshop on Earth System Analysis for Sustainability (2003)

Ages in Ga (109 yr BP)

Environment

Life

Complexity RevolutionOxygen RevolutionOrigin

Generic example of a tipping point

(System state)

(Sys

tem

sta

te)

Thanks to Chris Boulton for the animation

Tipping points in Earth history

PAL = Present Atmospheric Level

Lenton (2016) Earth System Science: A Very Short Introduction (OUP)

0100,000200,000300,000400,000500,000600,000Age (yr BP)

300

500

400

600

180200220240260280

Today’s CO2 Concentration

Projected Concentration After 50 More Years of Unrestricted Fossil Fuel Burning

CO

2[p

pmv]

Temperature proxy

Recent ice age cycles

0100,000200,000300,000400,000500,000600,000Age (yr BP)

300

500

400

600

180200220240260280

Temperature proxy

After 40 More Years of current energy use patternsC

O2

[ppm

v]

Today’s CO2 Concentration

Ice age abrupt climate changes

Deplazes et al. (2013) Nature Geoscience 6: 213-217

Age (thousands of years ago = ka)

Time

Temperature

in Greenland

Monsoon in

IndiaM

onso

on in

Am

azon

Smooth projections of future change

IPCC (2013) Working Group 1

IPCC AR5, WG1 SPM, 2013

Future worlds

Knutti et al. (2016) Nature Geoscience 9: 13-18

Climate tipping elements

Lenton et al. (2008) PNAS 105(6): 1786-1793

How close are tipping points?

Lenton & Schellnhuber (2007) Nature Reports Climate Change

Atlantic overturning circulation

Expert elicitation of probabilities• Three different warming scenarios:

• Example: collapse of Atlantic meridional overturning circulation

Kriegler et al. (2009) PNAS 106(13): 5041-5046

Greenland

Atlantic

Antarctica

Amazon

El Niño

Kriegler et al. (2009) PNAS 106(13): 5041-5046

Likelihood of tipping points

• 2-4 °C warming: >16% probability of passing at least one tipping point

• >4 °C warming: >56% probability of passing at least one tipping point

Abrupt changes in climate models

Drijfhout et al. (2015) PNAS 112(43): E5777-E5786

Tipping points of tropical tree cover

Hirota et al. (2011) Science 334: 232-5; Staver et al. (2011) Science 334: 230-2

Interactions between tipping events

Diebackof Amazonrainforest

Shift to a (more) persistent

El Ninoregime

Disintegration of West Antarctic Ice Sheet

Collapse ofAtlantic

thermohalinecirculation

Melt ofGreenland Ice Sheet

+

+

++

+

-

+ +

+/-

+/-

+/-

Reduced warming of Greenland

Cooling of NE tropical Pacific, thermocline shoaling, weakening of annual cycle in EEP

Enhanced water vapour export from

Atlantic

-

Heat accumulation in Southern Ocean

Southward shift of Inter-tropical Convergence

Zone

Drying over Amazonia

Tropical moisture

supply changes

Increase in meridional salinity gradient

Fast advection of salinity anomaly to North

Atlantic

Sea level rise causing grounding line retreat

Freshwater input

Warming of Ross and Amundsen seas

Increase in probability

Decrease in probability

Uncertain direction of change

+

-

+/-Tipping events are connected A→B if at least 5 experts judged that triggering A had a direct effect on the probability of triggering B thereafter

From expert elicitation: Kriegler et al. (2009) PNAS 106(13): 5041-5046

Thanks to Jonathan Donges and Ricarda Winkelmann, PIK, for the image

Early warning prospects

System beingforced past atipping point

Held & Kleinen (2004) GRL 31: L23207; Lenton et al. (2008) PNAS 105(6): 1786-1793

Thanks to Chris Boulton for the animation

Early warning indicator

Abrupt climate change early warningIce-coretemperatureproxy

Lenton, Livina, Dakos, Scheffer (2012) Climate of the Past 8: 1127-1139

Detrendeddata

Early warning indicator

Boulton, Allison, Lenton (2014) Nature Comms. 5: 5752

Early warning of a modelled tipping point

• Atlantic Meridional overturning circulation (AMOC) is currently monitored at 26oN

• In the ‘FAMOUS’ model we can collapse the AMOC and see early warning signals at 26oN

Early warning indicator

Where are the best early warning signals? How early are they?

Boulton, Allison, Lenton (2014) Nature Comms. 5: 5752

Red areas indicate early warning signals that are significant at p<0.05

Autocorrelation AR(1) Variance

Trends in sea surface temperature fluctuations

Red = slowing downBlue = speeding up

Boulton & Lenton (2015) PNAS 112(37): 11496-11501

Pacific decadal oscillation (PDO)

Autocorrelation

Variance

Boulton & Lenton (2015) PNAS 112(37): 11496-11501

Implications for marine ecosystems

Boulton & Lenton (2015) PNAS 112: 11496-11501, adapting the model of Di Lorenzo & Ohman (2013) PNAS 110: 2496-2499

Autocorrelation (AR(1)) of ocean variability

AR(1) of ocean variability

Ecos

yste

mst

anda

rd d

evia

tion

Ecos

yste

m

corr

elat

ion

with

oce

an

AR(1) of ocean variability

Rela

tive

impa

ct

Loss of Arctic summer sea-ice

Disintegration of West Antarctic ice

sheetIrreversible

meltdown of Greenland ice

sheet

Increase in El Nino amplitude

Collapse ofWest African

Monsoon

Collapse of Atlantic thermohaline

circulation

Dieback of Amazon rainforest

Dieback of boreal forest

Lowest risk

Highest risk

Lenton (2011) Nature Climate Change 1: 201-209

What should we do about tipping point risks?

Relative likelihood

Conventional cost-benefit analysis

Global temperature change

Pric

e

Cost of mitigation Cost of climate damages

But what if there are uncertain

tipping points?

Optimum (?)

Adding uncertain tipping points to a widely-used integrated assessment model

Lontzek, Cai, Judd, Lenton (2015) Nature Climate Change 5(4): 441–444

Representation of tipping points

Lontzek, Cai, Judd, Lenton (2015) Nature Climate Change 5: 441–444; Cai, Lenton, Lontzek (2016) Nature Climate Change

Tipping element Hazard rate (%/yr/K)

Transition time (years)

Final damages (% world GDP)

Atlantic overturning (AMOC) 0.063 10-50-250 10-15-20Greenland ice sheet (GIS) 0.188 300-1500-7500 5-10-15

West Antarctic ice sheet (WAIS) 0.104 100-500-2500 2.5-5-7.5Amazon rainforest (AMAZ) 0.163 10-50-250 2.5-5-7.5

El Nino (ENSO) 0.053 10-50-250 5-10-15

Cai, Lenton, Lontzek (2016) Nature Climate Change 6(5): 520–525

Tipping collective action• Uncertainty about the

location of a tipping point can lead to failure to coordinate to avoid it

• If uncertainty can be reduced below a critical level, social dynamics are tipped from free riding to coordinating to avoid catastrophe

Barrett & Dannenberg (2014) Nature Climate Change 4: 36-39

Tipping the energy system

Source: Michael Liebreich, Bloomberg New Energy Finance

Conclusions

• If business-as-usual continues then climate tipping points are expected to become high impact high probability events

• Tipping point early warning systems have been successfully tested and could be developed as an aid to adaptation to forewarn societies and trigger pre-emptive action

• The threat of multiple, interacting, uncertain climate tipping points should be triggering much stronger action to tackle climate change now, with a carbon price today of >$100/tCO2

Want to learn more?• 4 new Massive Open Online

Courses (MOOCs)– Climate Change: The Science– Climate Change: Solutions– Tipping Points– Valuing Nature

• FutureLearn• Early 2018• #ExClimate• @ClimateExeter

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