Lecture 2: Natural climatic variation and its causes (part 2) Flashcards
our records are limited
we are predicting the future based on present and past archives using:
modelling
observation
human archives
natural archives
Fluctuations in solar energy
See: graph in notes by David Chenette, Joseph B. Gurman, Loren W. Acton -
url=http://solar.physics.montana.edu/mckenzie/Images/The_Solar_Cycle_XRay_hi.jpg
David Chenette at Lockheed Martin, Advanced Technology Center., CC0, https://commons.wikimedia.org/w/index.php?curid=14896657
^ We are in a period of lower solar activity. Temperature would be expected to be cooling!
This is resulting in high visibility of aurora
The sun goes through a cycle of higher and lower activity every 11 years
^ Solar radiation and ejection of solar material
Appearance also changes due to:
- Solar flares - ‘coronal mass ejection’ result in visible aurora
- Sunspots – cool patches on sun ~size of earth visible as darker areas
- More sunspots -> More solar energy -> changes in aurora
This cycle is thought to be linked to climate, though mechanisms unclear:
- Total solar irradiance
- Cosmic rays -> cloud cover
Solar fluctuations in the past: sun spots
some periods of climate change in the past e.g. late 16-1700’s coincide with sunspots and aurora
^ see Eddy 1977
Historical records of sunspots, centuries old, may coincide with changes in climate
e.g. Little Ice Age (14th-19th C.) A period of cooling in N Hemisphere
Also reflected in 14C concentrations:
Lower solar activity -> higher cosmic rays -> more 14C produced
^ see stuiver and quay 1980
Oscillators e.g. El Niño – Southern Oscillation (ENSO) in the Pacific
Drive inter-annual to decadal climatic fluctuations
Does not cause persistent long-term climatic changes (temporary)
Can act as amplifiers or modulators of longer-term climatic shifts
Best known is the El Niño – Southern Oscillation (ENSO) in the Pacific
Operates on a 5-7 year cycle
See: https://www.metoffice.gov.uk/learning/learn-about-the-weather/what-is-el-nino-la-nina
Warm water collects in the West, resulting in upwelling of cooler water in East. Resulting in warm air upwelling on the west and cool air condensing in the East – weakening or reversing trade-winds
Main impacts flooding in South America e.g. Peru and droughts across Indonesia, India etc.
Each el nino is different but they are becoming more severe. It can also reverse resulting in la nina.
El Niño: upwelling off South America reduced or ceases; reduction in nutrient supply to surface water, and warmer SSTs, leads to crashes in fish populations, notably those of anchovies, with resultant impacts on populations of sea birds and mammals, as well as on the human population dependent upon the fisheries
We are currently in a ‘neutral position’ extreme flooding occurring in Spain and unusually temperate and green conditions in Kenya – still feeling the effects of el nino.
Global drifters: We know the temperature status across the worlds oceans due to the use of global drifters which communicate to satellites. They function for ~600 days and when they wash up they are returned to research centres.
El Niño: Southern Oscillation
El Niño years have anomalous rainfall patterns around the Pacific and beyond
*Irregular 5 – 7 year periodicity under present conditions
–SST records from δ18O of annually banded corals show periodicity and strength differed in the past
the water visibly piles up as precipitation distribution is disturbed
Oscillators in general
Oscillators
ENSO - only one of many similar oscillators in the climate system
e.g. North Atlantic Oscillation (NAO)
fluctuation in the pressure gradient between the Azores High and Icelandic Low alters the prevalent latitude of the track followed by storm systems crossing the Atlantic and entering Europe
alters wind strength over the Atlantic, in turn affecting the volume of warm surface water driven north-westward in the North Atlantic Drift
NAO fluctuations implicated in inter-annual to decadal fluctuations in e.g. composition of vegetation on a Gloucestershire road verge and in breeding success of Rissa tridactyla (Kittiwake) in north-east England.
Flights must be planned according to the jet stream that varies accordingly e.g. flights from NY to london vary dramatically in duration depending on ENSO wind patterns
Volcanic eruptions
Large eruptions are discrete, relatively uncommon and only the largest cause as much as 1°C of global cooling for a year or more (e.g. Krakatoa and others on the ‘Pacific ring of fire’)
Eruptions releases large volumes of dust and sulphate into upper atmosphere. This warms the stratosphere and absorbs incoming solar radiation
Resulting in cooling of the stratosphere and Earth’s surface
Pictured in notes: Mount Pinatubo, Philippines, erupting June 12th 1991
Resulting dust and sulphate aerosols in the stratosphere caused a global cooling of
ca. 0·5°C (temp reduction of 0.5 degrees C worldwide) that persisted for more than 2 years.
Volcanic eruptions example:
The Laki eruption Iceland 1783
*Emitted 122 megatons SO2 into atmosphere
*Annual mean surface cooling by -1.3°C in Europe and N America
*Subsiding plume led to severe volcanic pollution and acid rain
See changes in world temperature change around this period in:
Thordarson & Self (2003) J Geophysical Research 108: doi:10.1029/2001JD002042
Did volcanoes trigger the Little Ice Age?
14C in entombed plants used to model probability of ice growth at caps throughout 1200 yrs
Sulphates from several successive eruptions an alternative explanation for the Little Ice Age
Four large, sulphur-rich explosive eruptions triggered cooling
Cold temperatures maintained by sea-ice/ocean feedbacks (higher albedo= snow/ice increases reflectance of solar irradiance; colder ocean temperatures)
see Sulphate aerosol results from volcanic activity reflected in periods of ice growth Graphs from : Miller et al. (2012) Geophysical Research Letters 39: L02708, doi:10.1029/2011GL050168
if large volcanic eruptions cause decreases in global temperatures and glaciation, volcanic eruptions should have declined to result in higher temperatures - BUT They have not declined
Historical effects of humans on climate
Methane concentration in ice cores vary in near synchrony with precession pattern in solar radiation
CO2 concentration also varies with precession, obliquity and eccentricity cycles
BUT!!
Methane and CO2 concentrations appear to diverge from past patterns long before the Great Acceleration at the time of human agriculture
Identifying past human effects on CO2 and CH4 (methane)
Effects of human agriculture
-Increase in methane from increased Rice cultivation
see: L Ruddiman (2005) Scientific American March 2005, 42-53
-Increase in carbon dioxide from deforestation and burning (Eurasia) see: Ruddiman (2003) Climatic Change 61: 261-293.
Identifying past human effects on CO2 :
Greenhouse effect from CO2 and methane release from agriculture was enough to ward off glaciation in NE Canada:
see Ruddiman (2005) Scientific American March 2005, 42-53; Ruddiman (2003) Climatic Change 61: 261-293
Intergovernmental Panel on Climate Change (IPCC)
*Body founded in 1988 to provide policy-makers with a technical, scientific assessment of climate change, and our role in driving it
*Synthesis reports present the evidence-base from climate science of anthropogenic radiative forcing as the main driver of CC
*Presents future climate projections under different scenarios of greenhouse gas concentrations
We can use models to predict temperature changes with natural forcings only, and compare with predictions + anthropogenic forcings
With natural forcings alone, recent-past climate should have been fairly flat
With anthropogenic forcings, model prediction envelopes contain observed temperature/heat content
Future temperature projections, based on different CO2 concentration scenarios
Relative Concentration Pathways
-CO2-equivalent concentration of greenhouse gases that would result in (2046-2065): see table in notes
& see:; IPCC AR5 Greenhouse Gas Concentration Pathways Representative Concentration Pathways (RCPs) from the fifth Assessment Report by the International Panel on Climate Change
Summary
*As well as the many natural forcings acting on climate, there is evidence that humans have affected greenhouse gas concentrations in the past, leading to a climate that is different from that expected with natural forcings
*Using known natural forcings, we can pinpoint the most likely effects of current and future greenhouse gas concentrations on climate
*There is now overwhelming evidence that recent and ongoing climate change is being driven by anthropogenic forcings (CO2 and methane emissions)
*The IPCC 6th Report (2021) is online in both full report and synthesis/executive summary forms. You don’t need to read it all, but it’s worth having a look!
