Lecture 3. Evidence for climate change and climate variability

Maslin Chapter 3, Burroughs Chapter 8 and http://www.theresilientearth.com/?q=content/grand-view-4-billion-years-climate-change

Lecture structure

1. Evidence for Climate Change

2. Climate Change in Earth’s History

3. Climate and Human History

Background

In Lecture 1 we examined different types of climate variability, and we noted that many aspects of the climate system operate on different time scalesThe problem is to disentangle natural climate variability from warming due to enhanced greenhouse effect

A. Evidence for climate change: climate proxies

• Not possible to go back in time so use evidence remaining from those times, called ‘proxies’ to interpret palaeoclimate

Climate change proxies

• Tree rings – optimal conditions for a particular species result in more growth therefore thicker rings; events eg. fire, create scars

Climate change proxies

• Ice cores - eg from Lake Vostok, Antarctica or the Greenland Ice Sheet- gases and pollen trapped within layers infer past climate

Determining Past Climate Change using Oxygen Isotopes

• Normal oxygen contains 8 protons, 8 neutrons (O16)

• A small fraction (one in a thousand) of oxygen atoms contain 8 protons, 10 neutrons (O18)

• This is an isotope of oxygen and is heavier than O16

• O16 will evaporate more than O18 since it is lighter

• Hence, during a warm period, the relative amount of O18 will increase in the ocean waters since more of the O16 is evaporating

• Hence, looking at the ratio of O16 to O18 in ice cores the past can give clues about global temperatures.

Climate change proxies• Oxygen isotopes from micro-organisms found in

deep sea cores: use ratio of O16 and O18 – Foraminifera and diatoms (shelled micro-organisms with shells of

CaCO3) the O isotope ratio found in shell indicates past water temp since shells use heavier isotopes in warmer conditions

Shelled foramsDiatoms

Climate change proxies• Sediment cores- lake sediment layers indicate

sedimentation rate through time; remains of micro-organisms and pollen sink to bottom of lake and preserved in anoxic conditions; indicate past climate since each species has a limited habitable range

Tang Dynasty 600-900ADCapital XianChinese population 30mCultivated Rice and Wheat

Scientific controversy on climate/society

link

http://www.scmp.com/article/617611/climate-link-fall-tang-dynasty-questioned

Evidence for climate change: modern records

Automatic Weather Station network

Modern records: good accuracy and coverage

Variations of globally averaged surface temperature for 1850-2006 relative to 1961-90 Increase of 0.76oC +/-0.9oC Two warmest years are 1998 and 2005 Twelve of the 13 years 1995 to 2007 rank among the 13 warmest years on record Each of the 1st 8 months of 1998 was warmest of those months in whole record

Houghton p.71

Trend of annual mean temperature at Ta Kwu Ling 1989-2009

Increase 0.07 deg/decade

Annual mean temperature at HKO 1948-2009

Increase of 0.16 deg/decade

Increase of 0.28 deg/decade

Modern satellites• Only 30-40 years

record

Global av. sea level from tide gauge and satellite data (red) relative to 1961-90 meanOver 20th century sea level rose by 17+/-5cmRate of rise increased to 3.1cm for decade 1993-2003 - half of this due to thermal expansion of ocean, rest due to melting of glaciers

Evidence: modern recordsData from satellite images showing contraction of Greenland Ice cap

Combination of data proxies: indicates natural climate variability vs. recent change

B. Climate in earth’s historyLife appeared on earth when climate warmed at end of Pre-Cambrian period ca. 570MyaPhanerozoic (age of visible life) is the present period,Jurassic period of dynosaurs (approx. 100m ya) was a warm period of Mesozoic, when atm. CO2 5 times higher than today and no polar icecapsThen, a 65m yr transition to colder conditions, culminating in glaciation of Antarctica 35m yr ago accompanied by falling atm CO2 levels, Over the last 2.5Ma, (Pleistocene)ice ages have alternated with warmer periods and >80% of time colder than today

Phanerozoic: Period of visible life

(Burroughs Figure 8.5)

Past climate change: recentWe are at ‘0’ on lower graph- so we are currently in an ‘Ice-House’ world. Both poles still glaciated- rare in history of earth. Significant because of potential melting. Also large temperature gradient (+30 to -35º) gives us a dynamic climate system.

Life appeared Hominidsdynosaurs

Pleistocene Ice Ages

Between 2.5m and 1m years ago, ice ages occurred every 40k yrsLast 1 million years, ice ages every 100k yrsDue to changes in earth’s orbit related to sun

Last Glacial

Maximum 20ka ago

All of Canada

Greenland

UK

Last 10k yrs (Holocene) is a warm interglacialEnd of last ice age, temperatures rose by 6º in less than 4000 yrs, sea level rose 120m, atmospheric CO2 inc, by 1/3 and methane doubled

Asian shelf at Last Glacial Maximum (LGM) (18,000 ya)

Artemisia grassland in semi-arid environment

Southeast Asia during Pleistocene

glacial periods

The HoloceneHolocene (interglacial)- climate not constantAncient civilisations developed in Holocene Climatic Optimum 3000BCCooling cycles have occurred, with global mean temperatures cooling by 2ºC Influence on civilisations eg. 2000Ya collapse of many classical civilisations eg. Egyptian KingdomMediaeval warm period followed by “Little Ice Age” (Mediaeval Cold Period)

Holocene temperatures (BP)

Egypt collapse (slide 34) – defeat by Rome

Summary

• Although we are at a cool period in the long-term graph we are at a warm period of the Pleistocene (short term – last 2m. Yrs)

• Warming of 4-7ºC since LGM ten times slower than 20th century warming

• For last 150 years of reliable data records, the 12 warmest years occurred since 1996

Mediaeval Warm period 1000yr ago

Selection of temperature reconstructions – more available in later period

Black line=instrumental record

Little Ice age 1500-1700 AD• Norse colonies on Greenland collapsed• Famine, mass migrations in Europe• Ice fairs on R. Thames

http://trickygirl.wordpress.com/2009/12/01/the-little-ice-age-and-londons-frost-fairs/

Little Ice Age: Burroughs p.285-6

• Exceptional cold of 1690s killed half population of Finland – short summers destroyed crops

• Scotland- oats harvest failed and death rate 1/3 – 2/3 of population: Union of Scotland with England 1707 inevitable

• Irish famine in 1845 population of 8 million (1.5 million died and 1 million emigrated) due to wet climate and summer of 1845 especially wet with high humidity – ideal conditions for potato blight to spread

Obama’s State of the Union address 21.01.15

• “2014 was the planet's warmest year on record. Now, one year doesn't make a trend, but this does -- 14 of the 15 warmest years on record have all fallen in the first 15 years of this century.

• I've heard some folks try to dodge the evidence by saying they're not scientists; that we don't have enough information to act. Well, I'm not a scientist, either. But you know what -- I know a lot of really good scientists at NASA, and NOAA, and at our major universities. The best scientists in the world are all telling us that our activities are changing the climate, and if we do not act forcefully, we'll continue to see rising oceans, longer, hotter heat waves, dangerous droughts and floods, and massive disruptions that can trigger greater migration, conflict, and hunger around the globe. The Pentagon says that climate change poses immediate risks to our national security. We should act like it.”