Key Concept

s in Climate ScienceData collection and

presentation by Carl DenefJanuari 2014

*

The present slide show introduces the reader to the main aspects of the climate system and its natural variability. The presentation relies on what Climate Science has concluded from observed data and theoretical models. Climate Science is an integrative part of Earth System Science that aims to observe, interpret and explain the interconnection and balance between the four main Earth System domains: atmosphere, land, water and life. Climate Science requires the integrative participation of all scientific disciplines, going from physics, mathematics, geology, chemistry, statistics and computer sciences to biosciences, ecology, economy, sociology and anthropology. Climate Science is also related to human behavior and ethical consciousness, as human-induced climate change can have serious negative impacts on human prosperity and development, may worsen inequality between people and may trigger major conflicts.

Climate is the long-term average (at least 30 years) of the condition of the atmosphere in terms of temperature, humidity, atmospheric pressure, wind and precipitation in a given area. It represents a statistical distribution of these conditions, including the mean values and ranges.

The weather is the average condition of the various elements of the atmosphere in a given area over short periods.

Statistically significant deviations from the average condition are called anomalies or departures.

The climate system is an interactive system consisting of 5 major components: the atmosphere, the land surface, the hydrosphere (oceans, seas, rivers, lakes, aquifers), the cryosphere (land ice sheets,

sea ice, glaciers, snow fields) and the biosphere (living organisms, dead organic matter).

The climate system is complex and chaotic. Various weather and climate drivers and feedbacks exist. Their magnitude and the ways that these drivers and feedbacks interact are not fully predictable. Hence, climate variation and climate change can never be predicted with absolute certainty. Usually, predictions are formulated as a range between upper and lower values, with confidence limits and uncertainty evaluations.

The main drivers of climate are the incoming energy from the Sun and the reflection, absorption and re-emission of energy within the atmosphere, clouds and surface.

The climate system

◦ A climate in equilibrium is a dynamic equilibrium. At the global level total incoming energy is in balance with the total energy emitted to space, but not every location on Earth is in energy balance. The distribution of the net radiation imbalances over the globe are the origin of different types of climate. Imbalance brings the atmosphere and oceans into motion. Most of the incoming energy is captured in the tropics and subtropics, and then repartitioned to middle and high latitudes by winds and ocean flows.

◦ Any variation in the factors that affect incoming and/or outgoing energy or that modify the energy repartition, will affect climate. For some parts of the climate system the relationship between cause and response seems linear; in other

cases this relationship is more complex, characterized by hysteresis, or non-additive combination of feedbacks.

◦ Climate variability refers to changes in the energy balance over short time scales ranging from seasonal and annual, to a few decades. Some climate variations are cyclic while others are based on specific events, such as volcanic eruptions.

o Climate change refers to a change in the energy balance over time, scales ranging from decades to millions of years. Changes can be worldwide or only affect a region or a hemisphere. Responses may vary between regions. Climate change may also affect its variability and weather extremes.

◦ Various climate types are determined by lattitude, terrain, altitude, prevailing wind direction and nearby mountains and seas.

Source

Natural climate variability

◦ In addressing climate change it is essential to have insight into natural climate variability. Trends in climate change can then be distinguished more accurately and studied in climate models. Variability is often cyclic with a typical periodicity of a few years or a few decades. Variability may temporarily exacerbate or mask climate change.

◦ Climate variation can be seasonal or non-seasonal. The 4 season cycles (spring, summer, autumn and winter) are typical for higher latitude regions, while the cycles of rainy (monsoon) and dry season are seen in tropical regions.

◦ Several non-seasonal climate variations, most of them being cyclic, have been identified. Some will be shortly dealed with in the next slides (in order of importance):

:

• El Nino/Southern Oscillation

• North Atlantic Oscillation

• Pacific Decadal Oscillation

• Atlantic Multidecadal Oscillation

Seasons

◦ Seasons result from cyclic variations in the length of exposure to solar radiation, as a consequence of the tilt (23.5°) of the Earth's axis relative to the plane of the Earth’s orbit around the Sun. For a specified location the length of exposure (length of the day) depends on the time in the year.

◦ The time of the year when day and night have identical durations at all points of the World is called equinox. This occurs two times in a year: at the start of spring and at the start of autumn. The boundary between the illuminated and dark half-globes then passes through both Poles. The time of the year when daylight is longest on the northern hemisphere, is called June solstice. The time of the year with the shortest daylight in the northern hemisphere is called December solstice. The higher the latitude, the longer the longest day and the shorter the shortest day. Within the Arctic Circle daylight is all day long at the start of the summer and is absent at the start of the winter. In the Southern hemisphere, there it is winter during the June Solstice.

El Nino Southern oscillation (ENSO)

◦ ENSO is an oscillating variations in sea surface temperature (SST) in the east-central tropical Pacific Ocean (at least 0.5°C averaged over that region). A warming is known as El Niño and cooling as La Niña. This anomaly happens at intervals of 2-7 years, and lasts 9 months to 2 years.[5] El Niño is Spanish for "the child", and refers to Jesus, because initiation of warming in the eastern Pacific occurs usually around Christmas.[4]

◦ During El Niño SST is generally 1.5-2.5oC above average (up to 29 °C). In the subsurface (~100 m depth) ocean temperatures typically is 3o-6o above average. During La Niña SST is generally 1o-2oC and sub-surface temperature typically 2o-4oC below average.

◦ The fluctuations in ocean temperatures are accompanied by even larger fluctuations in air pressure between Darwin, Australia (western Pacific) and Tahiti (central-eastern Pacific). Low atmospheric pressure tends to occur over warm water and high pressure occurs over cold water. During El Niño there is low atmospheric pressure over Tahiti, and this is accompanied by high atmospheric pressure over Darwin, Australia, while La Niña displays high pressure in the eastern Pacific, accompanied by low pressure in the western Pacific.[2][3]

◦ The pressure differences strongly affect the Pacific trade winds (dominant winds that blow from east to west (easterly winds)). El Niño weakens the strength of the trade winds in the eastern Pacific, and reverses the trade winds in the western Pacific. Trade winds intensify during La Niña, due to the higher atmospheric pressure over the eastern Pacific, forcing an abnormal accumulation of cold water into the central and eastern Pacific Ocean.

◦ Trade winds move water warmed by the sun toward the west, which

creates upwelling of deep ocean off the coasts of Peru and Ecuador. This brings nutrient-rich cold water to the surface, sustaining large fish populations, which in turn provide abundant food to sea birds. El Niño reduces the upwelling of this nutrient-rich water, leading to fish kills off the shore of Peru with serious negative socio-economic impacts.[7]

Low pressure

Normal

El Niño

High pressure

La NiñaLow pressure

The strength of ENSO is quantified by the SST index and by the Southern Oscillation Index (SOI). SOI is computed from fluctuations in the surface air pressure difference between Tahiti and Darwin, Australia. [8] El Niño episodes are associated with negative values of the SOI, meaning there is below normal pressure over Tahiti and above normal pressure over Darwin (red color in Figure). Positive values are typical for La Niña (blue color in Figure).

Notice the domination of El Niño in the 1990s and of La Niña after 2000.

Source: Waish et al. Polar Science 7, 188-203, june 2013

SS

T d

ep

art

ure

°C

La Niña

El Niño

La Niña

El Niño

◦ El Niño SST index is monitored in four regions along the equator:

Niño 1 (80°-90°W and 5°-10°S)Niño 2 (80°-90°W and 0°-5°S)Niño 3 (90°-150°W and 5°N-5°S)Niño 4 (150°-160°E and 5°N-5°S)Niño 3.4 (120°-150°W and 5°N-5°S): correlates better with the SOI and is the prefered region to monitor SST.

Darwin, Australia

Sou

th-Am

erica

Look at an animation of the ENSO condition of the present year here (NOAA Physical Oceanography Division).

◦ During El Niño it rains more during the spring even in western Europe. During La Niña the opposite occurs. These climate oscillations cause extreme weather (floods and droughts) in many regions of the World. El Niño and La Niña also have a strong effect on the Jet Stream (read more)

◦ El Niño of 1982-83 brought extreme warming to the tropical Pacific. SST in some regions rose 6° C above normal. The warmer waters had a devastating effect on marine life off the coast of Peru and Ecuador. Fish catches were 50% lower than the previous year. The 1982-83 El Niño also had a pronounced influence on weather in the equatorial Pacific region and World wide. Severe droughts occurred in Australia, Indonesia, India and southern Africa. Dry conditions in Australia resulted in a 2 billion dollars loss in crops, and millions of sheep and cattle died. Heavy rains were experienced in California, Ecuador, and the Gulf of Mexico.

◦ What is suprising, however, is that the changes in SST are usually not large, plus or minus 3°C and generally much less, and yet, these minor changes can have large effects on global weather patterns and the biosphere.

The Pacific decadal oscillation (PDO)

◦ The PDO is a pattern of change of sea surface temperature in the Pacific Ocean north of 20° N. The PDO is detected as warm or cool surface waters. During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs. It shifts phases on a time scale of about 20 to 30 years.

Observed monthly values for the PDO (1900–feb2013).

North Atlantic Oscillation (NAO)

◦ NAO is a climate variation without periodicity in the North Atlantic Ocean, consisting of fluctuations of the difference in atmospheric pressure between the Iceland low and the Azores high pressure zones. The relative strengths and positions of these pressure zones oscillate in an east-west motion. A large pressure difference at the two locations is denoted as a high index NAO year (NAO+) and leads to increased westerly winds and, consequently, cool summers, and mild and wet winters in Central Europe and the Atlantic coast. If the index is low (NAO-), westerlies are suppressed, and these areas suffer more extreme climate (heat waves, deep freezes and reduced rainfall) and storm tracks toward southern Europe and North Africa.

The frequent negative NAO index values in the sixties coincide with the very cold winters in Europe at that time. The winter of 2009-10 in Europe was unusually cold which coincided with an exceptionally negative phase of NAO.[11] The very positive values in the late 80-ties up to 1995 corelate with very mild winters in that period.

Source

The Atlantic Multidecadal Oscillation (AMO)

◦ AMO is a cyclic variability in the sea surface temperature occurring in the North Atlantic Ocean. Instruments have observed AMO cycles only for the last 150 years. However, palaeoclimate studies of tree rings and ice cores, have shown that oscillations similar to those observed instrumentally have occurred for at least the last 1000 years.

◦ The AMO has affected air temperatures and rainfall over much of the Northern Hemisphere, in particular North America and Europe. It is associated with changes in the frequency of North American droughts and severe Atlantic hurricanes.

◦ Read more here (NOAA)

Climate forcings and feedbacks

◦ Climate can change through two categories of processes:

◦ Climate forcings: processes that are the primary drivers of climate change

◦ Climate feedbacks: processes that can either amplify or diminish the effects of climate forcing. A feedback that increases an initial change is called a "positive feedback”. A feedback that reduces an initial change is a "negative feedback".

Climate forcings Internal forcing

• Life processes• Thermohaline circulation• Tectonic plate activities

External forcing• Total solar irradiance• Cosmic ray irradiance• Variations in the Earth’s orbit• Radiative forcing• Aerosols, dust, smoke, and soot

Climate feedbacks• Cloud-albedo• Ice-albedo• Land use changes

Climate forcing◦ Life processes: Changes in plant mass distribution can force climate change, since

plants use CO2 and CO2 is the principal natural greenhouse gas that determines the temperature of the atmosphere at the Earth’s surface. An example of biological forcing is the ‘Azolla event’ (see Palaeoclimate)

◦ Thermohaline circulation in the oceans. Thermohaline is derived from ‘thermo’ - referring to temperature and ‘haline’ - referring to salt content of oceans, factors which determine the density of sea water. Wind-driven surface water currents (such as the Gulf Stream) travel polewards from the equatorial Atlantic Ocean, cooling en route, and eventually sinking at high latitudes as a consequence of wind-driven evaporation which makes the water more salty and cooler and hence more dense. This dense water then flows over the ocean basins to the Southern Oceans where Antarctic ice-cooled water sinks down. Because of these sinking cold waters, there must be water upwelling elsewhere. Thermohaline circulation causes extensive mixing between the ocean basins. The water masses transport heat around the globe which has a potential impact on climate. For example, in the North Atlantic, the northward flowing warm surface currents, including the Gulf Stream, provide heat of >1000 terrawatt to NW-Europe, which is equivalent to the energy output from more than one million 1-Gigawatt nuclear power plants.

The oldest waters have a transit time of around 2000 years. This ‘slow’ circulation nevertheless moves and mixes 900,000 Gigatons of water per year, equivalent to 30 times the global river flow. This ‘ocean conveyor belt’ also transports many Gigatons of carbon and nutrients which are vital for life. [Ref]

Thermohaline circulation

Source

◦ Plate tectonics. The outer layer of the Earth (the lithosphere) is broken into pieces, called tectonic plates, that move in relation to one another at a rate of about 3 cm per year. Over the course of millions of years, this motion configures where land and ocean are located on the globe (continental drift). The locations of land and sea affect the transfer of heat and moisture across the globe, and the location of land will affect the heat captured from the Sun, in this way determining climate. Continents warm at rates different from that of oceans, which determines global wind patterns .

When landmasses are concentrated near the polar regions, there is an increased chance for snow and ice to accumulate on land. Small changes in solar energy can then tip the balance between summers in which the winter snow mass completely melts and summers in which the winter snow persists until the following winter. The presence of snow and ice strongly increases albedo (surface reflectivity of solar radiation), resulting in cooling.

Uplift of land masses (for example of Himalayas by the Indian plate) leads to major weathering and sequestration of CO2 as carbonate in rocks

◦ Solar radiation

Solar radiation is the source of heat for the Earth. The Sun emits X-rays, ultraviolet (UV), visible light, infrared, and even radio waves, spanning a range of 100 nm to 1 mm waves.

Solar radiation at the top of the atmosphere is composed of ~ 50% infrared light (700 nm -1 mm), ~ 40% visible light (380-780 nm), and ~ 10% UV (100-380 nm).[3] At ground level it is ~ 53% infrared, ~ 44% visible light and ~ 3% UV.[4]

Total solar radiation (or irradiance) (TSI) at the top of the atmosphere is the amount of solar energy incident on an area with the Earth’s diameter and perpendicular to the rays. It is expressed in Watt (W)/m2 (the so called ‘solar constant’). At present, TSI = 1361 W/m2. However, taking into account that the Earths surface is a globe and that at any one moment half the planet does not receive any solar radiation, average solar radiation at the top of the atmosphere = ~ 340 W/m2. This value has been confirmed by recent SORCE/TIM satellite measurements (see IPCC- AR5 chap. 2).

There are cyclic variations in TSI with different periods.

1. The 11-years sunspot cycles[31]. There is 0.1% variation in the sun’s energy output with an average cycle of ~ 11 years, due to a cyclic expansion of the number of what is called ‘sunspots’. Sunspots are magnetic regions on the Sun surface with magnetic field strengths thousands of times stronger than the Earth's magnetic field. They are darker (emit less energy) than the surrounding area, but the surrounding margins - called faculae - are much brighter, resulting in more solar irradiation when sunspots are more numerous. A Sunspot typically lasts only for several days, but it is their average number that shows cyclic variation. Sunspots cycles have been studied and measured for several hundreds of years. Since 1979 changes in radiation have also been measured by satellites. It was found that the number of sunspots correlates with the intensity of

solar radiation. For the period 2001–2010, about 124 W/m2 of TSI was reflected by clouds, atmosphere and surface and 79 W was absorbed by the atmosphere (see IPCC- AR5 chap. 2). Therefore, global average TSI at the Earth’s surface = ~160 W/m². Thus, averaged over the globe, sunspot variation theoretically represents only ~0.16 W/m2 at the Earth’s surface.

2. Centennial cycles. As far as based on palaeoclimate reconstruction studies using cosmogenic radionuclides as proxies*, TSI shows cycles of 88 years (Gleisberg cycle), 208 years (DeVries cycle) and 1,000 years (Eddy cycle).

3. At a time scale of tens to thousands of years there are fluctuations of TSI that are associated with changes in the solar magnetic field. Evidence of variations in solar activity with this timescale can be found in the records of cosmogenic radionuclides.

* Proxies are preserved physical characteristics of the past that are in close correlation with the climatic conditions that prevailed during much of the Earth's history. Changes in their quantity allows reconstruction of climate changes up to 500 million years ago. Examples are: ice cores, relative quantities of particular isotopes, cosmogenic radionulides, tree rings, sub-fossil pollen, boreholes, corals, and lake and ocean sediments. Read more in the ‘Palaeoclimate’ section.

◦ Cosmic ray irradiance. Cosmic rays may be indirectly implicated in climate change because they produce ionized particles upon passage in the atmosphere that could serve as condensation centers of cloud formation. Clouds enhance Earth’s reflectivity of solar radiation and in this way result in surface cooling.

◦ Variations in the Earth’s orbit. The Earth’s orbit around the Sun is an ellipse. The Sun is positioned in one of the focal points. There are three variable components in this orbit: eccentricity, the axial tilt angle of the Earth's axis of rotation, and precession of the Earth's axis (see Figures in next slides and animations). These orbital alterations are known as Milankovitch cycles.

SEE ANIMATIONS

Eccentricity (or ellipticity) is expressed as % deviation from a circular orbit around the Sun. The maximum ellipticity is 6 %, the minimal 0.5 % (near circular orbit) and, at present, is 1.7 %. The distance of the Earth to the Sun determines the amount of energy received. It changes with the position of the globe on its orbit. At present, the closest distance (perihelion) is reached around January 3, the largest (aphelion) around July 4.

At present, the difference in TSI between perihelium and aphelion is 92 W/m² or a ~7 % difference in energy delivery (from Wikipedia). This is almost an order of magnitude higher than the difference in maximum and minimum TSI within the 11 year solar cycles today.

The eccentricity varies in cycles with periods of ~100,000 and ~413,000 years due to the gravitational influence of the Moon and other planets.

.

When the Earth’s orbit is at its most elliptical, the Earth in Perihelium is at its most closest to the Sun and, thus, will receive more heating at that position than when the orbit is at its most-circular. Thus, the level of eccentricity will affect climate.

The 100,000-year eccentricity cycle corelates with the Ice Age cycles during the last million years, suggesting that solar forcing is the dominant forcing signal.

Circular orbit, no eccentricity.

Elliptic orbit with 50% eccentricity.

Tilt angle is the angle between the Earth’s rotational axis and the axis of its orbit around the Sun. It slowly changes between 22.1° and 24.5° and back again, taking approximately 41,000 years to shift. When tilt increases, the amplitude of the seasonal differences in solar irradiation increases, with summers in both hemispheres receiving more radiative flux, and winters less. Conversely, when tilt decreases, summers receive less and winters more.

22.1–24.5° range of Earth's tiltSee animation

Importantly, these changes are not of the same magnitude everywhere on the Earth's surface. At high latitude the annual mean irradiation increases with increasing tilt, while lower latitudes experience a reduction in irradiation. Cooler summers are expected of facilitating the onset of an Ice Age, due to less melting of the previous winter's frozen precipitation. Currently the Earth is tilted at 23.44°, roughly halfway between its extreme values. The tilt is in the decreasing phase of its cycle, and will reach its minimum value around the year 11,800 . This tends to make winters warmer and summers colder (i.e. milder seasons), and cause an overall global cooling.

Because changes in winter and summer compensate for each other, the change in the annual average insolation at any given location is near zero, but the redistribution of energy between summer and winter strongly affects the intensity of seasonal variation. Such redistribution changes are considered a likely cause for the coming and going of recent Ice Ages.

Precession is a slow and continuous change in the orientation of the Earth’s rotational axis as referred to a fixed star (at present the North star). It is caused by tidal forces exerted by the Sun and the Moon’s gravity on the Earth’s equator. A full precession cycle has periods ranging between 19,000 to 24,000 years.

Effect on climate: When the Earth’s rotation axis points toward the Sun when the Earth is in perihelion, the northern hemisphere has a greater difference between the seasons while the southern hemisphere has milder seasons. When the axis points away from the Sun in perihelion, the southern hemisphere has a greater difference between the seasons while the northern hemisphere has milder seasons.

Precession See animation

See animation of a spinning top

The 100,000-year eccentricity cycle corelates with the Ice Age cycles during the last million years, suggesting that solar forcing is the dominant forcing signal. However, there is one problem: when tested by climate models the eccentricity variations have a significantly smaller impact on solar forcing than forcing by precession or tilt changes. Climate change is much more intense than the solar irradiance alone can explain. Therefore, various internal characteristics of the climate system are believed to be sensitive to the irradiation changes, which in turn amplify (positive feedback) or damp responses (negative feedback). See next slide.

◦ Radiative forcing. Radiative forcing is a measure of the disruption a factor has on the balance of incoming and outgoing energy in the Earth’s atmosphere and is expressed in W/m2. It is caused by an increase of greenhouse gas levels above the natural flux. In the context of climate change, radiative forcing is restricted to the near-surface troposphere.

◦ Greenhouse gases are water vapor, clouds, CO2, methane (CH4), nitrous oxide (N20), ozone (O3) and human-made halocarbons. The Earth’s surface absorbs the shortwave radiation (= visible, near-ultraviolet, and near-infrared waves) of the Sun and reflect a part back upward as longwave infrared radiation. Greenhouse gases absorb certain wavelengths of the long wave infrared light emitted by the Earth’s surface, which causes tropospheric heating, and then re-radiate it as longwave radiation (from 0.7μm to 5.0μm) in all directions, which causes additional warming of the troposphere (positive forcing). In addition, pollutants, such as CO, nitrogen oxides and SO2, enhance the greenhouse effect by altering the abundance of CH4 and ozone.

◦ Aerosols, dust, smoke, and ‘black carbon’ (soot). These are very

small particles suspended in the atmosphere. Sulfate aerosols are generated by burning coal and biomass and are present in volcanic eruptions. They tend to cool the Earth (negative forcing) because they absorb, scatter and reflect solar radiation. Other kinds of particles such as black carbon have a positive forcing by absorbing radiation and re-emitting it as longwave infrared light. Indirectly, aerosols also affect cloud albedo, because many aerosols serve as cloud condensation nuclei (CCN) or ice nuclei, which are initiators of cloud or ice formation. Changes in particle types and distribution can result in small but important changes in cloud albedo (see next slide). The global distribution of aerosols is being tracked from the ground and from satellites.

Climate feedbacks

◦ Cloud-albedo. Albedo of an object is the reflective capacity that object has on solar radiation. Clouds reflect ~1/3 of the sunlight back into space. Even small changes in the extent of clouds, their location and type have large consequences. Climate warming causes more water to evaporate, leading to an increase in cloudiness, which results in a cooling effect. On the other hand, water vapor and clouds are greenhouse gases causing warming. The net effect on surface temperature will depend on the cloud type and characteristics, the cloud height, and the nature of cloud condensation nuclei. Thin high clouds (height >6 km, paricularly cirrus clouds in the stratosphere) generally generate net heating, and thick low clouds (altitudes below 2 km) produce a cooling.

◦ Ice-albedo. Ice is white and very reflective, in contrast to the ocean surface, which is dark and absorbs more heat. As the atmosphere warms and sea ice melts, the darker ocean absorbs more heat, causes more ice to melt, and makes the Earth warmer overall. The ice-albedo feedback is a very strong feedback. The Earth's average surface is currently ~15 °C. If the Earth was frozen entirely, the average temperature would drop below −40 °C.[10] If all the ice on Earth were to melt, the average temperature on the Planet would rise to ~27 °C.[12]

◦ Land use changes. Land use changes by conversion of forests to agriculture, change the characteristics of vegetation, including its colour, seasonal growth and carbon content, which in turn affect CO2 flux and land-albedo.

Solar forcing vs climate feedbacks

◦ The 100,000-year eccentricity cycles corelate with the Ice Age cycles during the last million years, suggesting that solar forcing is the dominant forcing signal. However, when tested by climate models the eccentricity variations have a significantly smaller impact on solar forcing than forcing by precession or tilt changes. Climate change is much more intense than the solar irradiance alone can explain. Therefore, various internal characteristics of the climate system are believed to be sensitive to the irradiation changes, which in turn amplify (positive feedback) or damp responses (negative feedback). See ‘Climate change’ slides.

FORCINGS AND FEEDBACKS IN THE CLIMATE SYSTEM

Black carbo

nLWR

SWR=shortwave radiationLWR=longwave radiation

From IPCC AR5 (adapted) OVERVIEW