GPHY 314 Chapter 2: Is the Climate Changing?

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29 Dec 2020
CHAPTER 2: Is the Climate Changing?
Primary statistic will be global average temperature because…
o Most direct impact from addition of GHG to atmosphere is increased temperature
o We have the best data for temperature -> Technology is centuries old
Rather than analyze temperature directly, analyze temperature anomalies (difference between
temperature and reference temperature) and reference is usually average temperature over
previous multi-decadal period
Use anomalies rather than absolute because…
o Absolute temperature can vary sharply over short distances, while anomalies are constant
over much longer distances i.e. regions of warm and cold anomalies tend to be
hundreds/thousands of kilometers across
Calculations of global average temperature anomalies require only about a
hundred or so temperature stations spread across the globe
o Can measure changes in a quantity even if you cannot measure the absolute value
Recent Climate Change: Surface Thermometer Record
Use automated electronic thermometers to estimate global average surface temperature anomaly
of earth over past 150 years -> Data clearly show that the Earth is warming
o From 1880 to 2012, average surface temperature rose by 0.85°C
Not uniform, occurred primarily in two periods: 1910-1945, 1976-2002
o Recent warming is basically continuous, with every decade since the mid-twentieth
century warmer than previous decades
Three warmest years were 2005, 2010, 2014
Year-to-year variations in global average temperature quite small; just a few tenths of a degree
Warming of twentieth century distributed across planet, occurring just about everywhere
o Not entirely uniform; most obvious difference that land warmed more than oceans
o Northern hemisphere warmed more than tropics or southern hemisphere, tropics and
southern hemisphere warmed about equally
Surface thermometer record wasn’t designed for climate monitoring and has undergone many
changes i.e. types of thermometer used, station location and environment, observing practices,
other sundry alterations all can introduce spurious trends in data
o Scientists know about these problems, adjust data to take them into account
o To account for uncertainties in data, scientists use error bars (estimate of potential
error in their estimate) on the trend
For warming from 1880 to 2012 of 0.85°C, error bar is 0.20°C, meaning
warming is very likely between 0.65°C to 1.05°C
Recent Climate Change: Satellite Measurements of Temperature
Possible to measure global average temperature from orbit and US has been flying instruments on
satellites to make that measurement since 1978
o Data show general warming trend over this period of approximately 0.14°C per
decade/1.4°C per century
As with all data sets, has own set of problems and uncertainties:
o Satellites actually measure average temperature of lowest 8km of atmosphere, from
surface to altitude where airliners fly
Not actually measurement of surface temperature
o Orbital drift of satellites carrying instruments leads to flying over location later each day
Temperatures rise through day, so generally warmer in location at 3pm than 2pm
Drift itself would introduce warming trend, even if climate not changing
o Calibration of satellite instruments, which weren’t designed for long-term measurements
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o Shortness of satellite record (just a few decades long)
One way to gain confidence in satellite and surface thermometer records is compare them
o Excellent agreement between these two independent measurements provide strong
confirmation of reality of warming seen in both data sets
Sometimes lots of shorter-term ups and downs superposed on long-term warming trends which
are assigned to physical causes -> Mainly volcanic eruptions and El Niño-La Niña cycles
o During El Niño events, earth warms several tenths of a degree Celsius
o During La Niño events, earth cools several tenths of a degree Celsius
o ^Cause temporary fluctuations in temperature lasting a few years but no long-term
changes in the climate
o Volcanic gases emitted during eruptions cool climate by blocking sunlight
After few years, effluents removed from atmosphere and climate normal again
Recent Climate Change: Has Global Warming Stopped?
One claim that earth was warming but it stopped 5/10/15 years ago, so nothing to worry about
Sources of short-term variability do not have anything directly to do with climate change and
don-t cause any long-term changes in climate
o Bumps and wiggles make determining trends over short periods i.e. decade, problematic
Possible to generate continuous set of short-term cooling trends even as climate experiencing
long-term warming i.e. start trend calculation during El Niño year (hot) and end it in La Niña or
volcanic year (cool)
Term for this deceptive argument: “Going down the up escalator”
Claiming global warming has stopped requires careful selection of endpoints
o Cherry picking: intentionally selecting data to yield result counter to full data set
Only possible to find cooling over short time periods
o Over several decades, long-term warming dominates and even more egregious endpoint
selection can’t generate cooling trend
Recent Climate Change: Ice
Since ice melts reliably at 0°C, is a dependable indicator of temperature
If warming trend is correct, should expect to observe ice disappearing
Glaciers: form in cold regions when winter snow doesn’t completely melt during next summer
o As snow accumulate over millennia, snow at bottom is compacted by weight of overlying
snow and turns into ice, eventually producing glaciers
o Length, areal extent, total volume of glaciers have been monitored for decades/centuries
Glaciers began retreating ~1800 with recession accelerating later in 19th C
Pattern consistent worldwide, confirming warming is global
o Decreases in precipitation/cloudiness can also cause glaciers to recede but no evidence of
global trends in either one that could cause the reduction in glacier lengths
But do have evidence of global trends in temperature
Sea ice: in polar regions, seawater freezes to form layer of ice floating on top of the ocean,
typically few meters thick
o Area covered varies, reaching maximum in late winter, minimum in late summer
o Given rapid warming in Arctic, see reductions in area covered by sea ice during summer
Winter temperatures so low, a few degrees of warming don’t have much effect
In addition to shrinking in area, sea ice also thinner
o Sea-ice area around Antarctic remained stables since mid-1970s
o Overall pattern of large loss in Arctic but little loss in Antarctic matches regional
temperature trends in these regions, which show large rapid warming in northern
hemisphere and weaker warming in southern hemisphere
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