Desonie D. Climate

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Climate

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increased spring and summer temperatures and earlier snowmelt.

Similar changes in wildfire increases have been seen in other parts of

the Americas.

Wrapup

The effects of climate change are already apparent in living systems.

Plants and animals are changing their ranges and the timing of their

life cycles. As a result, some populations have expanded, some have

decreased, and some have just shifted. If these strategies do not work,

the organisms will die out locally, or the species may even become

extinct. The loss of a species that is important in a food web, such as

krill, can have repercussions throughout an ecosystem. The conse-

quences of warming that have been seen so far are just the beginning

of the changes that are predicted to come as temperatures continue

to rise.

A WARMER FUTURE

PART THREE

he scientific consensus is that global warming is now under

way. It is now necessary to determine how much, when, and

what the consequences will be. As Dr. Robert Watson, then

Chairman of the Intergovernmental Panel on Climate Change (IPCC),

said in 2001, “The overwhelming majority of scientific experts, whilst

recognizing that scientific uncertainties exist, nonetheless believe that

human- induced climate change is already occurring and that future

change is inevitable.”

With the use of climate models, scientists are attempting to develop

scenarios that predict the magnitude and timing of climate change.

future Climate models

To understand future climate scenarios, scientists construct complex

climate models. Good climate models take into account paleoclimate

data, thermal inertia, the effects of oceanic and atmospheric currents,

and the cooling effects of sulfate aerosols, among many other factors.

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of Global Warming

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Complex models require the use of very powerful computers to make

these calculations.

Climate models have many uncertainties: Among the most impor-

tant of them is the difficulty of predicting how humans will behave in

the future. The models cannot predict, for example, how much carbon

people will emit into the atmosphere. Will CO

emissions increase at

the same rate they have for the past decade? Or will they increase at

an even higher rate due to improvements in lifestyles in the develop-

ing world (and even further improvements in the developed world)?

Or might they decrease due to conscious changes in lifestyle or tech-

nology? Because human behavior is unpredictable, climatologists

construct models using different estimates for these numbers, ranging

from conservative to extreme. For example, a climatologist might input

an annual reduction in carbon emissions into a conservative model or

allow CO

emissions to grow rapidly using an extreme model. After the

calculations are made, the results will be very different.

Models of future climate look at the effects on the Earth’s system

of various increases in greenhouse gas levels or in temperature. The

simplest of them are called commitment models, which predict the

responses that are inevitable due to greenhouse gases that have already

been added to the atmosphere. A commitment model predicts the

amount of warming that will occur as thermal inertia is overcome, even

if people stop adding greenhouse gases to the atmosphere. For example,

the temperature commitment for greenhouse gases released as of 2000

is a rise of between 0.9° and 1.8°F (0.5° and 1.0°C) by 2100. Com-

mitment models are extremely unrealistic because they assume that

people will immediately cease emitting greenhouse gases. Their value

is in providing a baseline picture of the inevitable changes to come.

Many models of future climate assume that greenhouse gas levels

will be stabilized at a temperature or a CO

level that society chooses.

These models require that people choose a value and actively reduce

greenhouse gas emissions to reach it. Some modelers use a doubling

of atmospheric CO

from its preindustrial value of 280 ppm. A recent

report says that a CO

value of 560 ppm will cause average global

temperature to rise up to 5°F (2.8°C).

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The doubling model does not take time into account, so there is

no time frame for when this doubling will occur. To determine when

this CO

level will be reached, a rate of change of CO

values must

be figured into the model. One commonly used rate of change is an

increase in CO

by an extra 1% per year. In this model, CO

doubles

over preindustrial values by about 2080. Indeed, a value of 560 ppm

by 2080 is not unlikely.

The business-as-usual model assumes that emissions will rise

along the same trajectory they have been on for the past decade, and

that there will be no efforts to reduce greenhouse gas emissions. At

this rate, CO

emissions in 2015 will be 35% greater than they were in

2000, and in 2030 they will be 63% greater than they were in 2002.

he business-as-usual model results in CO

levels over 600 ppm and

a temperature increase of 0.9 to 3.6°F (0.5 to 2°C) by 2050. (Thermal

inertia would keep the temperature from rising higher.) By 2100, CO

could reach 880 to 1,000 ppm, a level not seen for at least 30 million

years, with a certain temperature rise of between 3.5 and 8°F (2 and

4.5°C) and a 10% chance of an even greater rise.

Most models stop at the year 2100 or at a doubling of CO

, but

one business-as-usual model by scientists at the Lawrence Livermore

Laboratory in California was allowed to run out to 2300. This model

calculated what would happen if people continued to use fossil fuels

until the Earth’s entire supply was exhausted. The model run began in

1870 and predicted a fairly accurate increase in temperature of 1.4°F

(0.8°C) by 2000. By 2300, there was a quadrupling of CO

levels

from preindustrial to 1,423 ppm, and global surface temperatures were

14.5°F (7.8°C) higher than today. Land and polar areas warmed more,

with the most extreme warming of more than 36°F (20°C) taking place

over the Arctic. The oceans absorbed a great deal of CO

and became

more acidic, with the potential of harming marine life. As seawater

temperature surged, much of the ocean’s dissolved CO

was driven

back into the atmosphere, which further enhanced warming. The

ost drastic changes came in the twenty-second century, when green-

house gas emissions rates were the highest and the environmental

changes—in precipitation, extent of sea ice, and other features—were

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the greatest. Ice and tundra gave way to forests, and Northern Hemi-

sphere sea ice cover disappeared almost completely by 2150.

The study’s lead author, Govindasamy Bala, recognizes some weak-

nesses in the model but says that correcting the weaknesses would

actually make the situation more drastic. The model assumes that soil

and living biomass on land are carbon sinks but does not take into

account a few factors such as the clearing of forests. In a 2006 press

release about his study, Bala said, “We definitely know we are going

to warm over the next 300 years. In reality, we may be worse off than

we predict.”

In the alternative scenario, people quickly and severely limit

greenhouse gas emissions. CO

emissions level off by the end of this

decade, slowly decline for a few decades, and begin to decrease rapidly

y mid-century. Depending on the magnitude of the greenhouse gas

emission reduction, this model predicts an increase of less than 2°F

(1.1°C) this century.

The models discussed above are global models, but local models

can be constructed as well. Global models leave out some very impor-

tant information. A good example is the effect of global warming on the

Arctic. A global average temperature increase of 5°F (2.8°C) means a

ne- or two-degree increase at the equator but a 12°F (6.7°C) increase

at the North Pole. This has profound implications for melting sea ice

and ice caps, for polar ecosystems, and for the climate system due to

the positive feedback of reduced albedo.

Global dimming may be a larger factor in the accuracy of climate

models than anyone realized until recently. Most climate models use

the link between CO

levels and past temperature to predict how tem-

perature will respond to future CO

levels. But scientists now say that

warming in recent decades has been partially counteracted by global

dimming. In that case, climate models consistently underestimate the

temperature effects of rising CO

if global dimming is reduced (due to

pollution reductions). For example, when a climate model that predicts

a 9°F (5°C) temperature increase by the end of the century has a reduc-

tion in global dimming factored in, the predicted temperature increase

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rises to 18°F (10°C). Therefore, unless greenhouse gas emissions are

also curbed, a decrease in air pollution will increase global warming.

While the timing and magnitude of changes due to rising tem-

perature cannot be known with certainty, climatologists agree that all

models point in the same direction. Another point of agreement is that

climate change will not continue to be gradual; when thresholds are

broken, climate will change rapidly.

Different models of climate change predict different temperatures by 2100, depending

on whether or how much greenhouse gas emissions are restricted. The averages of

the different models are included in the gray area; the colored shading encompasses

projected growth ranges for the twenty-rst century.

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Climate ConsequenCes by temperature

The IPCC’s Fourth Assessment (IPCC-4) report, from 2007, shows the

responses that can be expected for various increases in temperature

above 1990 levels:



Up to 1.8°F (1°C): Some ecosystem shifts; most coral reefs

bleached; some increases in global agricultural production

potential, but reduced yields at lower latitudes; increases in

severe storms



From 1.8 to 3.6°F (1 to 2°C): About one-quarter of spe-

cies lost from current range; further increases in global

agricultural production potential but further yield reduc-

tion at lower latitudes; coastal flooding; increased water

stress in dry areas, with increased risk to human health;

increased frequency and intensity of drought; increased

heat waves



For 3.6 to 5.4°F (2 to 3°C): Most of tundra and about half

of boreal forest area disappears; about one-third of species

lost from current range; all coral reefs bleached; global

agricultural production potential peaks, bringing hunger

to populations in lower latitudes; 1 billion people become

water stressed



For 5.4 to 7.2°F (3 to 4°C): Global decreases in agricultural

production potential; large numbers additionally at risk of

unger; up to one-third of global population water stressed;

widespread deglaciation and disintegration of West Antarc-

tic Ice Sheet; further increased intensity and frequency of

fire, drought, and storms



For 7.2 to 9.0°F (4 to 5°C): Decreases in agricultural

production potential at higher latitudes, as well as further

decreases at lower latitudes; too expensive to protect many

areas from flooding; frequency of hot days much greater,

many locations very difficult to live in; weakening and shut-

down of some ocean currents, including Atlantic meridi-

onal overturning

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

For 9.0 to 10.8°F (5 to 6°C): Widespread species

extinction.

The following sections describe in more detail what climate models

predict for the future as temperature rises.

temperature patterns

Temperatures will rise as atmospheric greenhouse gas levels increase,

but temperature increases will lag behind greenhouse gas levels due to

thermal inertia. Temperatures will not increase uniformly around the

globe: Most lands will warm more than the oceans due to the thermal

inertia of the seas; the Northern Hemisphere will warm more than the

Southern because it is mostly land; and the polar regions will warm

much more than the rest of the planet due to the transition of ice to

liquid water.

Water CyCle

The water cycle will continue to intensify. Precipitation will decrease

in drier areas, including in many mid-continental areas, and increase

in wetter areas. Rainfall will decline in some locations in the sum-

mer, which will harm crop yields, increase forest fires, and increase

water stress. Currently, about 1.7 billion people live in countries

that are water stressed: By 2025, that number will likely grow to

5 billion. Droughts will become longer and fiercer, making many

marginal regions difficult or impossible to inhabit, including the

western United States, northern China, and southern Africa. Deserts

will become drier, and the Asian monsoon will be enhanced but will

likely become more variable.

arCtiC

The Arctic will continue to melt. As ice is lost, animals such as polar

bears and seals will become greatly reduced in number or will go

Future Consequences of Global Warming