This giant, complex conveyor belt moves water from the surface to deep oceans and back.
North Atlantic Deep Water (NADW): a cold, deep-water current that's part of the MOC.
North Atlantic Deep Water (NADW)
Salty water from the Gulf Stream moves northward on the surface in the high-latitude North Atlantic.
The water becomes denser by cooling from Arctic air currents and evaporation, increasing salinity and density.
This denser water sinks, forming the NADW.
The NADW spreads south through the Atlantic to South Africa, where it is joined by cold Antarctic waters, which stream north into the Indian and Pacific Oceans as deep currents.
Thermohaline Circulation's Impact on Climate
The movement of warm water to the North Atlantic transfers enormous amounts of heat toward Europe, providing a much-warmer-than-expected climate.
Ocean water circulation is extremely complex, but turnover does occur, forming deep waters at the poles and warm water affecting Europe’s climate.
Large amounts of freshwater lower water’s density.
Melting ice or precipitation entering Arctic waters could slow or stop thermohaline circulation.
The North Atlantic Oscillation (NAO) affects wind and storms across the Atlantic.
The Interdecadal Pacific Oscillation (IPO) is a warm-cool cycle over the Pacific, occurring over several decades.
El Niño/La Niña Southern Oscillation (ENSO) reverses trade winds over the equatorial Pacific Ocean.
ENSO dominates global climate for a year or two.
El Niño leads to warming, La Niña leads to cooling.
The 1997–2000 ENSO cost $36 billion and killed 22,000.
Climate Change Science
Global and regional climates do change.
Today, humans are changing the climate with warmer air and oceans, increased precipitation and extreme weather, melting glaciers, and rising seas.
The IPCC report states it is very likely (90%–100%) that most warming is due to human-induced increases in anthropogenic greenhouse gases.
Carbon dioxide rose 40%, largely from 1960, from 280 ppm to over 400 ppm due to burning fossil fuels, agricultural wastes, deforestation, and other sources.
The Field of Climate Change
Climatologists study more than just the climate, including the atmosphere, ocean currents, ice, and rocks.
Climate change is a relatively young field that uses new technologies.
Its large scale requires government funding and tracking across national boundaries.
The Intergovernmental Panel on Climate Change (IPCC) was formed in 1988 to consolidate and synthesize research, providing consistent data to scientists.
IPCC and UNEP
The UN Environmental Program and the World Meteorological Society founded the IPCC in 1988 to provide accurate information leading to understanding human-induced climate change.
IPCC's Objectives:
Risk assessment: how much is the climate changing and how do these changes affect Earth and us?
How can we adapt and mitigate climate change?
The IPCC contains three working groups:
Working Group I assesses scientific issues.
Working Group II evaluates impacts and adaptation.
Working Group III investigates ways of mitigation.
Availability of Expert Information
Climate Change 2014: Synthesis Report is the IPCC’s AR5 report (2013–2014).
Over 850 authors from 85 nations worked for free.
It includes a summary and longer reviews of the latest scientific literature on climate change.
The American Association for the Advancement of Science (AAA) published What We Know (2014) to inform the public.
The U.S. Global Change Research Program published National Climate Assessment 3.
Climates in the Past
It is harder to find evidence of climate change the further back in time we go.
Records of weather, temperature, precipitation, storms, etc. have only been kept for about 130 years, since 1880.
Since 1880, Earth has warmed and cooled.
Earth warmed from 1910–1945 and from 1976 to the present.
Since 1976, each decade warmed 0.28^\circ C (0.5^\circ F) over land and 0.11^\circ C (0.20^\circ F) over the ocean.
Every year since 1976 has had an average global temperature higher than the long-term average.
Proxies for Past Climates
Proxies: Records providing information on climate from much further back in time, using temperature, ice cover, precipitation, tree rings, pollen deposits, and landscapes.
Also from marine sediments, corals, and ice cores.
The Northern Hemisphere warmed from AD 1100 to 1300.
Little Ice Age: 1400–1850.
Ice cores show that climate can change in decades, using CO2 and CH4 (methane), isotopes of O and H.
Information from Ice Cores
Climate shifts between ice ages and warm periods.
Ice ages tie up water in glaciers, lowering sea levels.
8 glacial periods occurred over the past 800,000 years.
Ice ages have lower levels of greenhouse gases and temperatures, which match exactly.
Carbon dioxide (CO_2) levels ranged between 150 and 280 ppm.
Milankovitch cycles: Climate changes due to variations in Earth’s orbit in intervals of 100,000, 41,000, and 23,000 years.
Rapid Changes in Climate
When are rapid climatic fluctuations superimposed on the major oscillations?
During glaciation and warmer times
The Younger Dryas event: 11,700 years ago, at the end of the last ice age.
Dryas: a genus of arctic flower
Arctic temperatures rose 7^\circ C in 50 years.
Caused enormous impact on living systems.
Warming was not caused by changing solar output.
Triggered by a change in ocean currents, volcanoes, or an impact or burst of air from a near-Earth comet.
Evidence of Climate Change Today
Weather varies naturally year to year.
Local temperatures may not follow global averages.
Average global temperature has risen 0.6^\circ C (1.1^\circ F) since the mid-1970s (0.2^\circ C/decade).
Warming is happening everywhere, greatest at high latitudes of the Northern Hemisphere.
This warming is caused by an enhanced greenhouse effect by anthropogenic greenhouse gases.
Oceans Are Warming
The upper 3,000 meters of the ocean have warmed, dwarfing the warming of the atmosphere (90% of the heat increase in Earth’s systems).
Over the last decade, oceans have absorbed most of the heat not yet seen in the atmosphere.
Long-term consequence: as this stored heat comes into equilibrium with the atmosphere, it will increase atmospheric and land heat even more.
Short-term consequence: Thermal expansion and melting glaciers and ice caps have caused sea levels to rise 15 cm (6 in.).
Glaciers and Ice Caps Melting
The IPCC AR5 reports a loss of land ice from glaciers and ice caps thinning and melting.
Sea ice is also melting but does not cause sea level to rise (except through thermal expansion).
Sea level was stable until 100 years ago.
Sea rise is a new phenomenon consistent with the known pattern of anthropogenic climate change.
Other Noted Changes by IPCC AR5
Increased warm temperature extremes.
Decreased cold temperature extremes.
Heat waves are increasing in intensity and frequency, such as in Europe (2010), California, and Australia (2013–2014).
Droughts are increasing in intensity and frequency, leading to fewer crops and more forest fires.
Changing patterns of precipitation and flooding, with greater amounts from 30º N and S poleward.
More frequent flooding, e.g., in the U.S. Northeast and Midwest.
Further Changes Reported by IPCC
Spring comes earlier, fall later, in the Northern Hemisphere.
Ecosystems and populations are out of sync.
Tree deaths and insect damage.
Alaska, Siberia, and Canada have warmed 5^\circ F in summer and 10^\circ F in winter.
Spring comes 2 weeks earlier than 15 years ago.
Melting permafrost is lifting buildings, uprooting telephone poles, and breaking up roads.
Additional IPCC Findings
Rising Arctic temperatures have shrunk Arctic sea ice.
Positive feedback loop: melted ice exposes dark water, which absorbs energy, melting more ice.
Unprecedented melting of the Greenland Ice Sheet, with the largest loss in 2012–2013.
It could raise ocean levels 23 feet.
Antarctica temperatures have risen 0.5^\circ C.
The West Antarctic Ice Sheet (WAIS) is below sea level and is shrinking as water eats away at it.
Further Changes Reported by the IPCC
Accelerating glacier melting since 1990.
Mountain glaciers are critical sources of freshwater.
Bursting dams cause floods and landslides and kill people.
Marine fish populations have shifted northward, impacting commercial fisheries.
Ocean acidification due to CO_2 absorption is killing coral reefs, changing the surface ocean’s chemistry.
Rising Greenhouse Gases
IPCC AR5 Report: It is extremely likely that over 50% of temperature increases are due to human increases in greenhouse gasses.
Greenhouse gas emissions from humans continue to increase.
The most important gas is carbon dioxide (CO_2).
Over 100 years ago, Swedish scientist Arrhenius predicted that burning fossil fuels may increase CO_2, but he was not concerned about the impacts.
Measuring Carbon Dioxide Levels
In 1958, Charles Keeling began measuring CO_2 levels on Mauna Loa, Hawaii.
Atmospheric CO_2 levels have increased 1.5–2 ppm/yr.
Seasonal changes in photosynthesis and respiration cause seasonal variations in CO_2 levels.
Respiration (fall through spring): CO_2 increases.
Photosynthesis (spring through fall): CO_2 decreases.
In 2014, CO_2 levels were 400 ppm, 40% higher than before the Industrial Revolution, higher than in the past 800,000 years.
Rising Carbon Dioxide Levels
Burning fossil fuels increases CO_2 levels.
1 kg of fossil fuel burned produces 3.7 kg CO_2.
10 billion tons (gigatons, Gt) of fossil fuel carbon (GtC) are released each year—a 2.5% increase.
Half of CO_2 comes from industrialized nations.
5% comes from cement production (but is included with fossil fuel emissions).
CO_2 accounts for 20% of greenhouse warming.
Climate change skeptics claim CO_2 comes from natural sources; however, data suggests otherwise.
Sources of Carbon Dioxide
Skeptics suggest that CO_2 comes from natural sources.
To determine the source we can look at radioactive isotopes of carbon
^{14}C (a radioactive isotope) is formed in the atmosphere.
Fossil fuels do not have ^{14}C.
^{14}C is clearly decreasing.
Deforestation adds 0.9 GtC/year.
Over the past 50 years, release of carbon has tripled.
Half of the carbon is removed by sinks.
Carbon Sinks
Burning fossil fuels should add 3 ppm/year to the air, but only 2 ppm/year is actually added.
Carbon sinks (the ocean, biota) absorb CO_2.
Oceans are a sink for 30% of the CO2 emitted, taking up CO2 by phytoplankton or undersaturation.
Limitations exist to oceanic CO_2 absorption.
Only the top 300 m of the ocean contact the air.
Ocean acidification will reduce CO_2 absorption.
Terrestrial ecosystems are also carbon sinks, storing 15% of the CO_2.
Forests, especially, can sequester carbon.
Other Increasing Greenhouse Gases
These gases absorb infrared radiation and add to the effects of CO_2.
Most are anthropogenic sources and are increasing.
Water exists as water vapor and clouds, accounting for 50% (water vapor) and 25% (clouds) of the greenhouse effect; it is the most abundant greenhouse gas.
Its tropospheric concentration varies but is rising.
It undergoes rapid turnover in the lower atmosphere.
Methane heats 25 times more effectively than CO_2.
Microbial fermentation of plants in wetlands and ruminants.