The science of climate change

Daniel J. Jacob

Vasco McCoy Family Professor of Atmospheric Chemistry and Environmental Engineering

School of Engineering & Applied Sciences, Dept. of Earth & Planetary Sciences

Harvard University

Part 1: The drivers of climate change

Part 2: The impacts of climate change

THE EARTH’S CLIMATE IN EQUILIBRIUM:�BALANCE BETWEEN INCOMING AND OUTGOING ENERGY

SOLAR

RADIATION

(visible)

​

​

28% reflected by clouds, ice,

aerosol particles… (albedo)

TERRESTRIAL

RADIATION

(infrared)

Effective temperature

T_E = 255 K (-18^oC, 0^oF)

Emitted radiation

varies as 4^th power of temperature

WARMING OF EARTH’S SURFACE BY GREENHOUSE GASES

SOLAR

RADIATION

(visible)

​

TERRESTRIAL

RADIATION

(infrared)

Greenhouse gases are transparent to solar radiation but absorb

terrestrial infrared radiation and re-emit it both upward and downward

​

28% reflected by

clouds, ice…

Water, CO₂, methane are the most important greenhouse gases

Atmospheric

greenhouse gases

Effective temperature

T_E = 255 K (-18^oC, 0^oF)

Surface temperature

T_o = 288 K (15^oC, 59^oF)

Solar

Terrestrial

visible infrared

F_in

F_out

increase greenhouse gas by ΔG

F_in

F_out

Climate equilibrium: F_in = F_out

ΔG

Radiative forcing ΔF = F_in – F_out > 0

Climate change arises from disruption of this radiative equilibrium

increase albedo by ΔA

F_in

F_out

ΔA

Radiative forcing ΔF = F_in – F_out < 0

visible infrared

visible infrared

positive radiative forcing

warming

negative radiative forcing

cooling

🢡

🢡

radiative energy fluxes

Radiative forcing of climate change drives cascade of impacts and feedbacks

Temperature response ΔT_o

Physical impacts

• sea level rise
• ice loss
• precipitation changes
• floods and droughts
• fires, ecosystem changes

Societal impacts

• water resources
• public health
• agriculture, fisheries

climate

feedbacks

triple ΔT_o

ΔT_o ∝ ΔF

Surface temperature response

is proportional to radiative forcing

transition to new climate

…until tipping points

ΔT_o = 1.5-4^oC

Tipping point from the recent past: glacial-interglacial climate transitions�

glacial

glacial

glacial

glacial

Vostok ice core (East Antarctica)

Abrupt climate change driven by initial solar radiative forcing

strongly amplified by water, ice, CO₂ feedbacks

current interglacial

Today: 430 ppm

The natural carbon cycle

volcanoes

erosion

atmospheric CO₂

600 Pg C

land carbon

3,000 Pg C

ocean carbon

40,000 Pg C

sediment carbon

90,000,000 PgC

burial

warming drives CO₂ evasion: positive feedback

1 Pg = 1x10¹⁵ g = 1 billion tons

photosynthesis

respiration

fires

dissolution evasion

Increase in atmospheric CO₂ from fossil fuel combustion

IPCC AR6; https://www.esrl.noaa.gov/gmd/ccgg/trends

CO₂ , parts per million (ppm)

May 2024: 427 ppm

growth rate: 3 ppm per year

50% of added CO₂ stays in atmosphere; rest is taken up by land (25%) and oceans (25%)

2023 emissions:

10 Pg C per year

Increase in atmospheric CO₂

= 5 Pg C per year

volcanoes

erosion

atmospheric CO₂

600 Pg C

land carbon

3000 Pg C +130

ocean carbon

40,000 Pg C

sediment carbon

90,000,000 PgC

burial

fossil fuel combustion

(x50)

~ 200 years

to take up CO₂

+ 260

+ 130

Perturbation of carbon cycle by fossil fuel combustion

Increase in carbon reservoirs since preindustrial time

Tipping point from the recent past: glacial-interglacial climate transitions�

glacial

glacial

glacial

glacial

Vostok ice core (East Antarctica)

Abrupt climate change driven by initial solar radiative forcing

strongly amplified by water, ice, CO₂ feedbacks

current interglacial

Today: 430 ppm

Acidification of the ocean from increasing CO₂

CO₂

H₂CO₃

HCO₃^_ + H⁺

Air

​

Ocean

Acidification of the ocean endangers marine biosphere by making it more difficult to form calcium carbonate shells

Change in ocean pH since preindustrial

WMO (2022), GLODAP

0.1 pH decrease

means 26% acidity increase

Methane is a major greenhouse gas… but where does it come from?

Methane: another major greenhouse gas

Wetlands: 100-220 Tg/yr

Livestock: 90-140

Oil/gas: 40-80

Waste: 50-80

Rice: 20-40

Coal mines: 20-60

Pre-industrial: 650 ppb

Present: 1930 ppb

Atmospheric lifetime: 9 years

Fine particulate matter (PM_2.5): major component of air pollution

http://www.nasa.gov/topics/earth

cools the Earth by reflecting solar radiation

combustion

industry

dust

aerosol particles (0.01-10 μm)

relative humidity

>100%

cloud droplets condense on particles

solar radiation

US air quality standard

Radiative forcing since pre-industrial time and temperature response

IPCC AR6

​

• Radiative forcings are additive and consistent with observed temperature rise, providing foundation for climate policy

Radiative forcing (W m^-2)

Temperature response (^oC)

radiative forcing from 1750 to 2019

surface temperature response

The agents of climate change

+1.2^oC

CO₂ (100+years)

methane

(9 years)

others

(~100 years)

aerosol

particles

(1 week)

Observed rise in global mean surface temperature

Warming from methane includes additional contributions from ozone, water, and CO₂ when methane is oxidized in atmosphere

IPCC projections of CO₂ emissions and resulting temperature

business-as-usual

aggressive decarbonization

+4.4^oC relative to 1850-1900

+1.4^oC

IPCC AR6

+1.6^oC

+2.4^oC

• Aggressive action is needed if we are to avoid 2 degrees of danger
• Getting CO₂ concentration to decrease requires cutting emissions by more than half

CO₂ emission (GtCO₂/year)

Recent trends in CO₂ emissions

GtCO₂/year

IEA [2024]

1^st IPCC report

Kyoto protocol

Paris agreement

2023:

+1.1%

2022-2023 increase is driven by global South; coal combustion accounts for 65%

PM_2.5 air quality may be major driver of decarbonization in global South�

…but this will offset the near-term benefit

of CO₂ decreases

State of the Air, 2020; Zhai et al., 2019

Annual mean PM_2.5 in China

2013

2018

action to decrease PM_2.5

Projected temperature responses from aggressive 1.5^oC decarbonization scenarios relative to constant emissions

Decreasing aerosol delays improvement until 2050

Shindell and Smith [2019]

No improvement until 2050

relative to constant emission scenario

Increasing attention to methane in climate policy

Biden at COP26 announcing Global Methane Pledge, now signed by 150 countries

“One of the most important things we can do to keep 1.5 degrees in reach is to reduce methane emissions as quickly as possible…it amounts to half the warming we’re experiencing today…We collectively commit to reduce our methane emissions by 30% by 2030 and I think we can go beyond that”

Why the importance of methane for staying below 1.5^oC of danger?

+1.2^oC

CO₂ (100+years)

methane

(9 years)

others

(~100 years)

aerosol

particles

(1 week)

Observed rise in global mean surface temperature

Effect of zeroing out methane emissions

Observed rise in global mean surface temperature

+0.7^oC

Warming decreases by 40%

CO₂ (100+years)

others

(~100 years)

aerosols

(1 week)

buys us time as we work to decarbonize the economy

Decreasing methane would also improve ozone air quality

with benefits for public health, crops, natural vegetation … and CO₂

Simple measures can go a long way, there is no stockage problem like for CO₂

fix leaks, venting practices

flare excess gas

…or use it

recover gas from landfills

digest gas from manure ponds,

wastewater plants

change rice practices

Decreasing methane emissions should be easy

change cattle feed

…but problem is that methane comes from a zillion �of individually small point sources with highly variable emissions

oil field in California

I’m leaking!

I’m venting!

My flare went out!

Satellite observations can tell us where methane is coming from

Balasus et al., 2023

…and most is coming from the Global South

TROPOMI satellite observations of atmospheric methane, 2021

coal

livestock

landfills

rice

livestock

landfills

rice

wetlands

livestock

rice

oil/gas

livestock

wetlands

landfills

livestock

landfills

oil/gas

oil/gas

Top ten methane emitting countries [Tg/year]

Qu et al., ACP 2021

Chen et al., ACP 2022

Nesser et al., ACP 2024

*

did not sign the Global Methane Pledge

*

*

livestock

rice

gas

coal

waste

oil

from inversions of GOSAT and TROPOMI satellite data

Satellites can observe large methane plumes �to enable immediate climate action

Pipeline

Pipeline blocking valve in Mexico

Q = 300 tons h^-1, 3-h duration

Watine-Guiu et al. [2023]

EELL pipeline from Chihuaha to Durango

supplying Permian gas to Mexico

blocking valve

Targeting methane emissions cannot be a substitute for CO₂… �because it does not address long-term climate change

​

• Over 10-year horizon, methane is more important than CO₂
• But over 100-year horizon, methane is long gone while CO₂ is still there
• Methane and CO₂ emissions should not be viewed as “equivalent” in climate policy (which they unfortunately are)

Response after 10 years

Response after 100 years

Warming response after 1-year pulse of present-day emissions (IPCC AR6)

CO₂ CH₄

Methane is still a powerful lever for near-term climate action�…while we decrease CO₂ emissions and develop carbon capture technologies

Time

Climate risks

CO₂ emission decrease

+ carbon capture

+ methane emission decrease

Start of climate action

(+solar geoengineering?)

Solar geoengineering

Science for SAI is OK but only as stopgap while we aggressively decrease CO₂

- it does not address ocean acidification

- it would cause catastrophic warming if used for decades then stopped suddenly

Part 2: The impacts of climate change

How to observe climate change

Paleorecords:

ice cores, tree rings, sediments

Long-term surface stations:

weather, gages, cruises

Earth-observing satellites:

atmosphere, (sub)surface

Many climate variables:

• Atmosphere: temperature, precipitation, clouds, …
• Land: surface type, soil moisture, river flow…
• Ocean: temperature, sea level rise, acidity…
• Ice: extent, thickness, albedo,…

How to model climate change: Earth system models (ESMs)

supercomputer

interpretation

projections

what-if scenarios

• Solve coupled equations for evolution of atmosphere, oceans, land, ice…
• The system is chaotic: the models give probabilities

• 134 models from 53 centers contributed to IPCC AR6 (CMIP6)
• IPCC uses ‘wisdom of crowds’ to make projections from CMIP6 model ensemble

A small rise in mean temperature �means disproportionally more heat extremes

• Heat waves become more commonplace
• New extremes are reached, potentially crossing dangerous thresholds

Temperature

Probability of occurrence

heat waves

average

Small change in average temperature

Large increase in heat waves

New extremes are reached

cold waves

Large decrease in cold waves

Extreme wet bulb temperature (TW) as dangerous threshold

extremely

hazardous

death in 6 hours

Raymond et al., Sci. Adv. 2020

T increase

TW is the temperature of air if you evaporated as much water in it as you could

Warming causes more intense rain events

Temperature [K]

Saturation water vapor pressure [hPa]

water vapor air capacity

Increases exponentially

with temperature

• Warmer air holds more water vapor, which gets released at once when rain occurs
• More intense rain events result in increased flooding

Precipitation trend 1901-2019

At the same time, droughts are expected to get worse

• The western US is getting drier:

- higher water evaporation not offset by precipitation

- diminished snowpack

- higher runoff from more extreme rain events

​

higher

precipitation

higher evaporation

Regions of worsening drought

due to climate change (IPCC AR6)

Land surface

Heat and drought have driven increased wildfires in western US

Abatzoglou et al. (2021)

New York City, June 7, 2023

Trends in smoky days, 2011-2020

• Attribution to climate change is complicated by legacy of fire suppression
• Fires are now the #1 source of PM_2.5 pollution in the western US
• Increasing fires have offset air quality gains

It’s warming everywhere – but most so in the Arctic

https://data.giss.nasa.gov/gistemp

• Positive feedback from sea ice loss, lower snow cover

Warming of Arctic slows down winds in the northern hemisphere

HOT

COLD

COLD

trade winds

westerlies

Equator

30^o

30^o

N Pole

S Pole

• Weaker thermal contrast between Equator and Arctic means weaker winds
• More stagnant weather means heat waves, cold spells, air pollution episodes
• Weather systems shift northward

​

Rapid loss in summertime Arctic sea ice

March 2022 September 2022

• Expect ice-free summer Arctic by 2050 (dialed in)
• Albedo feedback drives fast Arctic warming
• Other effects: biosphere, erosion, navigation…
• Arctic permafrost thaw could release large amounts of CO₂ and methane
• But winter ice is holding on

% change relative to 1991-2020 average

March

September

https://arctic.noaa.gov/; IPCC AR6

Melting of Antarctic and Greenland ice sheets

Total Antarctic ice mass: 30 million Gigatons (Gt) Total Greenland ice mass: 3 million Gt

Measured by high-resolution gravity data

from NASA GRACE satellite

Mean sea level rise

2010-2017 loss of ice

IPCC AR6

thermal expansion

global mean sea level

20 cm since 1900;

recent acceleration driven by melting of glaciers, Greenland

Projections of future sea-level rise

IPCC AR6

• Future rise is dialed in by irreversible melting of glaciers, Greenland
• Rate of rise is sensitive to emission scenario

Concern over possible collapse of Western Antarctic Ice Sheet

• The Western Antarctic Ice Sheet is being eroded from below by retreat of the grounding line
• Models indicate a tipping point below 2^oC
• Total collapse would raise sea levels by 7 m

Very obvious impacts of sea-level rise

Islands, low-lying areas become uninhabitable

4.9 million in US live less than 4ft above sea level

Kiribati

Hurricanes, coastal storms cause more damage

Weakening of Gulf Stream, increased ocean stratification

• As Arctic ice melts and surface ocean heats up, there is less deep water formation to drive the ocean circulation
• This could restrict CO₂ uptake by oceans and drive a major positive feedback

IPCC AR6

Heavy (cold and salty) water sinks to bottom of ocean, driving ocean circulation

The Earth has had many different stable climates over its history

Snowball Earth (700 million years ago)

Eocene (35 million years ago)

Cretaceous (100 million years ago)

Last glacial climate (20 thousand years ago)

How to switch from one stable climate to another

ENERGY

TEMPERATURE

Stable climate 1

Stable climate 2

Stable climate 3

Perturbation

​

variabilité

interannuelle

negative

feedback

positive

feedback

tipping point

Can be large and abrupt but reversible

Irreversible but can be slow

(100-10,000 yr)

CO₂ over past 60 million years:�highly correlated with climate transitions

IPCC AR6

Current estimates of climate tipping points

Armstrong McKay et al. [2022]

• Tipping points can be expressed n terms of global surface temperature increase since pre-industrial times, become increasingly likely as Earth warms
• We are already probably past some tipping points (Greenland ice sheet, coral reefs)