LLL Innis 2 2018

WHAT CONTROLS CLIMATE?

We know that in our solar system,

the Earth is a special place.

However, recent discoveries by the Kepler

Space Observatory suggest that there

may be a billion similarly located Earth-

sized planets just in the Milky Way, our

galaxy. All are at considerable distance.

The implications?

Our future, if we

have one, is here.

How and why is the Earth different from the

other planets?

1.It has an atmosphere composed predominantly of

nitrogen (78%) and oxygen (20%). It produces a modest

greenhouse effect because of small, but significant

amounts of carbon dioxide, methane and other gasses.

How stable is this composition?

How much has it changed through time?

What are the causes and consequences of changing

atmospheric chemistry?

2.It has lots of water (a hydrosphere). This water

can occur as liquid, solid and gas and readily

exchanges between the three states. Energy

exchanges accompany changes in state (e.g.

hurricanes as power plants) .

At present 70% of the Earth’s surface is water-

covered.

How has this changed through time?

What roles do oceans play in driving

environmental change?

3.The Earth has a biosphere. Much of it is

microbial.

How does the biosphere interact with

climate?

How does it contribute to the greenhouse

effect?

How has the biosphere changed through

time and why?

4.The Earth is dynamic and mobile, a

remnant of its origins. The processes are all

part of plate tectonics.

The rock record tells us that PT has been

going on for perhaps 4 billion of the Earth’s

4.5 billion year history.

The Earth’s internal processes generate a

strong magnetic field.

Our magnetic field, produced by the

internal workings of the Earth, protect us

from most harmful solar radiation.

The Earth has been called ‘The

Goldilocks Planet’ –not too hot, not

too cold, not too wet, not too dry; just

right.

The key is the availability of water

and its ability to exist in liquid,

gaseous and solid forms.

At one time, both Mars and Venus were in

the Goldilocks Zone. Both had surface

water. Venus has a runaway greenhouse

effect with a mean temperature of 460C; its

water long boiled away. Mars is now a

freeze-dried desert; its mean temperature is

-50C.

What drives climate?

Our Goldilocks status is clearly

determined two major constraints;

A. the supply of energy from the sun and

the modification imposed by distance from

it.

B. internal Earth processes, collectively

labelled plate tectonics.

Let’s first look at Energy Supply

Solar radiation and its behaviour in the Earth

system is determined in part by the nature of

that radiation, particularly its wavelength.

Incoming radiation has short wavelengths

mostly in the visible light part of the spectrum.

We assume a constant supply, the solar

constant, but it varies at a variety of scales.

Perhaps the best known short cycle is the 11

year sunspot cycle.

The role of Earth’s sphericity and

orbit

The amount and nature of energy received

depends on the intensity and duration of

solar radiation. This is determined by Earth

curvature and by the Earth’s axial

inclination. In general, intensity declines

from Equator to Poles. There is increasing

seasonality in receipt along the same

gradient.

Simply, the highest inputs occur at/close

to the Equator and decline towards the

poles.

Inclination and parallelism produce

seasonality.

Changing Earth-Sun geometry .

Further complications come from long-term

cyclical changes in Earth-Sun relationships.

These are (a) eccentricity of Earth orbit, (b)

cyclical variations in the angle of inclination,

and (c) the wobble of that axis (precession).

These are called Milankovitch Effects. They

operate at cycles of 26,000, 41,000 and 100,000

years and appear to be responsible for the

pulse-like ice advances and retreats that have

marked the last 1.5 million years.

Milankovitch Effects

The role of the atmosphere

Before it reaches the Earth’s surface, solar radiation

has to pass through our atmosphere. Here it can be

reflected, scattered or absorbed. The amount of

reflection is called albedo. It’s about 90% for fresh

snow and about 15% for boreal forest. The Earth’s

average albedo is 30%, but it’s increasing!

About 50% of incoming radiation gets directly to the

Earth’s surface. It gets reradiated in the longwave

(mostly infra-red). It’s this radiation which is largely

responsible for our temperature regimes; our daily

and seasonal cycles.

Our collective role has been to change the behaviour of the atmosphere by;

A. influencing Earth surface albedo by forest clearance, urbanization, etc., and atmospheric albedo mostly through dust production. This should cause global cooling.

B. changing its chemistry mostly through increasing the concentration of greenhouse gases (CO2, CH4,O3, etc.). This should produce global warming.

Another important influence is

atmospheric chemistry, particularly the

proportion of Greenhouse Gases.

Plate Tectonics and Climate Change

The internal processes of the Earth and their

surface expression are generally referred to

as plate tectonics. There have been several

cycles of agglomeration and dispersal since

PT began about 4 billion years ago.

The current cycle began with the dispersal of

the last supercontinent, Pangaea, over the

last 175 million years.

Tectonic plates

PT, volcanoes and earthquakes

Internal processes

Paleomagnetism and seafloor

spreading

Former supercontinents

Pangaea/Pangea – the last

agglomeration

50 million years ago

The world 250 million years

ahead

Plate tectonics and climate;

PT influences climate by (a) changing the

locations of continents, (b) changing their

topographies and by (c) influencing

atmospheric chemistry.

Continental location and topography determine

atmospheric circulation.

They also exert a major control on oceanic

circulation .

PT and the oceanic circulation

In the short term, PT operates most

obviously through pyroclastic dust

production and changes in atmospheric

albedo and chemistry. Most climatic

responses are rapid, but of short duration,

e.g. Tambora, 1815 and the Year

Without Summer.

The effects of single eruptions induce

cooling.

Tambora, 1815,

VEI 7

Mt. Pinatubo,

Philippines,1991

PT and the Greenhouse Effect;

In the long term, PT controls global climate

by determining the size of the Earth’s

Greenhouse Effect. This is always around

and is NOT a human artifact. Today it’s

modest; about 30C.

PT does this by its control of the carbon

cycle. Most of the Earth’s CO2 is stored in

rocks. PT cause C02 to be released or

stored.

Most of our carbon is stored in rocks. It’s

released by volcanic activity and stored

through erosion and sedimentation.

Global equilibrium

Despite the huge changes evident in the geologic record, Earth seems to be remarkably resilient. Over the long and short term, negative feedback maintains an equilibrium. We don’t have to buy into radical concept like James Lovelock’s Gaia Hypothesis to explain it.

Let’s look at the ways in which contemporary climate is kept in broad balance.

Earth’s curvature

determines an uneven

receipt of solar energy, but

Earth systems, notably the

atmospheric and oceanic

circulations transfer

energy from areas of

surplus to areas of deficit.

They are tightly connected by the constant

exchanges that take place between ocean

and atmosphere, particularly latent

energy exchange. Thus the energy

cycle and the hydrologic cycle are

inextricably linked.

Atmospheric circulation

The oceanic circulation has two components;

a surface circulation and a deep-water

(thermohaline) circulation.

Interactions between the two

circulations

The transfer of energy is continuous

and largely unnoticed except during

major transfer events such as

hurricanes.

A hurricane releases 70X the world

energy consumption per day; the

equivalent to exploding a 10

megaton bomb every 20 minutes !!!

Hurricane Irma, 2017

The patterns of energy transfer

vary annually, seasonally, weekly

and daily. The latter two are part of

that natural variability that we call

weather.

There are also some very important cyclical changes that impact globally. Most involve distinctive changes in atmospheric circulation. They include the Pacific Decadal Oscillation and the North Atlantic Oscillation, but the best known of these is the El Nino/Southern Oscillation (ENSO) phenomenon. We’ll look closely at ENSO later in the series.

Next week we will examine how climate

change has constrained the evolution of

life on Earth. For most of Earth history

life was mostly microbial (it still is!).

From the Cambrian, about 600 million

years ago, life expanded in both form

and function (the Cambrian Explosion).

How did climate drive this?