Global Warming
Part 1: The Long-Term Global Historical Temperature Record
The historical record of global temperatures comes from various sources since instrumentally recorded temperatures exist for just over 100 - 150 years and even then for many parts of the world the measurements are sparse.
The Intergovernmental Panel on Climate Change (IPCC) is a United Nations based group focused on evaluating global climate change. Figure 1-1 is from their latest report (2007). It shows their calculated global average temperature for 1850 to 2006. (See Part 3 of this Global Warming document for information on how global temperatures are currently estimated.)
Figure
1-1: Global Temperature Anomaly for 1850 AD to 2006 AD
[From
IPCC Climate Change 2007 Summary for Policymakers p 6]
From the Figure 1-1 graph it appears that the global average temperature has increased by approximately 0.8 degree over the last 150 years.
Since widespread,
reliable instrumental records are available only for the last 100 – 150 years
or so, climatic conditions in the more distant past are estimated by analyzing
proxy evidence from sources such as tree rings, corals, ocean and lake
sediments, cave deposits, ice cores, boreholes, glaciers, and documentary evidence.
Knowledge of past climates is necessary in order to put the recent climate into
a broader context. Starting in the late 1990s, scientists began combining proxy
evidence from many different locations in an effort to estimate surface
temperature changes averaged over broad geographic regions over various time
frames. The first of these reconstructions published in 1998 and 1999 by
Michael Mann, Raymond Bradley, and Malcolm Hughes, attracted considerable
attention because the authors concluded that the Northern Hemisphere was warmer
during the late 20th century than at any other time during the past millennium.
Controversy arose because many people interpreted this result as definitive
evidence of anthropogenic causes of recent climate change, while others
criticized the methodologies and data that were used.
Figure 1-2 shows average northern hemisphere temperature
anomalies for the last 1000 years and is known as the “hockey-stick” graph. The
recent global warming theme was initiated based on this “hockey-stick” graph
created by Michael Mann, Raymond Bradley, and Malcolm Hughes and published by the IPCC
in its Third Assessment Report (TAR) in 2001. (Part 2 of this Global
Warming document discusses this graph in some detail since this was
instrumental in creating the current politicized global warming situation.)
Figure 1-1 fits within this area of Figure 1-2
Figure
1-2: Northern Hemisphere Temperature Anomaly
for
1000 AD to 2000 AD
[From
IPCC TAR Technical Summary 2001 p 29]
Figure 1-2 shows an apparent slight cooling trend for most of the last 1000 years with a definite uptrend in the last 100 years. This leads to the assumption that it has anthropogenic cause since this period is the industrial age. The gray banding in Figure 1-2 indicates the levels of uncertainty in the data.
Figure 1-3 shows a similar time range as Figure 1-2 from a different source (National Academy of Sciences) – but it’s not exactly a hockey stick.
Figure 1-1 fits within this area of Figure 1-3
Figure
1-3: Global and/or Northern Hemisphere Temperature Anomalies
from
Proxy Data for the last 1000 years
[From:
Surface Temperature Reconstructions for the Last 2,000 Years
Committee
on Surface Temperature Reconstructions for
the
Last 2,000 Years, National Research Council]
http://dels.nas.edu/dels/rpt_briefs/Surface_Temps_final.pdf
There are many reconstructions of temperature records from various proxies, many of which exhibit quite different trends. Figure 1-4 shows a comparison of 10 different published reconstructions of mean temperature changes during the last 2000 years. It is unknown which, if any, of these reconstructions is an accurate representation of climate history; however, these curves are a fair representation of the range of results appearing in the published scientific literature. Jones and Mann have made an apparently conscious attempt to eliminate the Little Ice Age from the record.
Figure
1-4: Temperature Anomalies for the Last 2000 Years
(dark blue
1000-1991): Jones,
P.D., K.R. Briffa, T.P. Barnett, and S.F.B. Tett (1998). "High-resolution
Palaeoclimatic Records for the last Millennium: Interpretation, Integration and
Comparison with General Circulation Model Control-run Temperatures". The
Holocene 8: 455-471
(blue
1000-1980): Mann, M.E.,
R.S. Bradley, and M.K. Hughes (1999). "Northern Hemisphere Temperatures
During the Past Millennium: Inferences, Uncertainties, and Limitations". Geophysical
Research Letters 26 (6):
759-762.
(light blue
1000-1965): Crowley and
Lowery (2000). "Northern Hemisphere Temperature Reconstruction". Ambio 29: 51-54.; Modified as
published in Crowley (2000). "Causes
of Climate Change Over the Past 1000 Years". Science 289: 270-277.
(lightest blue
1402-1960): Briffa,
K.R., T.J. Osborn, F.H. Schweingruber, I.C. Harris, P.D. Jones, S.G. Shiyatov,
and E.A. Vaganov (2001). "Low-frequency temperature variations from a
northern tree-ring density network". J. Geophys. Res. 106: 2929-2941.
(light green
831-1992): Esper, J.,
E.R. Cook, and F.H. Schweingruber (2002). "Low-Frequency Signals in Long
Tree-Ring Chronologies for Reconstructing Past Temperature Variability". Science 295 (5563): 2250-2253.
(yellow
200-1980): Mann, M.E.
and P.D. Jones (2003). "Global Surface Temperatures over the Past Two
Millennia". Geophysical Research Letters 30 (15): 1820. DOI:10.1029/2003GL017814
(orange
200-1995): P.D. Jones
and M.E. Mann (2004). "Climate Over Past Millennia". Reviews of
Geophysics 42: RG2002.
DOI:10.1029/2003RG000143
(red-orange
1500-1980): Huang, S.
(2004). "Merging Information from Different Resources for New Insights
into Climate Change in the Past and Future". Geophys. Res Lett. 31: L13205. DOI:10.1029/2004GL019781
(red 1-1979): Moberg, A., D.M. Sonechkin, K.
Holmgren, N.M. Datsenko and W. Karlén (2005). "Highly variable Northern
Hemisphere temperatures reconstructed from low- and high-resolution proxy
data". Nature 443:
613-617. DOI:10.1038/nature03265
(dark red
1600-1990): Oerlemans,
J.H. (2005). "Extracting a Climate Signal from 169 Glacier Records". Science 308: 675-677. DOI:10.1126/science.1107046
(black 1856-2004):
Instrumental data was jointly compiled by the Climatic Research Unit and the UK
Meteorological Office Hadley Centre. Global Annual Average data set
TaveGL2v [2]
[from global warming art http://en.wikipedia.org/wiki/Image:2000_Year_Temperature_Comparison.png ]
Figure 1-5 shows eight records of temperature variability on multi-centennial scales throughout the Holocene period (period since the last ice age), as well as an average of these (thick dark line). The records are plotted with respect to the mid 20th century average temperatures, and the global average temperature in 2004 is indicated. This figure can not resolve temperature fluctuations finer than approximately 300 years. The inset plot compares the most recent two millennium of the average to other high resolution reconstructions of this period.
Figure
1-5: Temperature Anomalies for the Last 12000 Years
[from global warming art http://en.wikipedia.org/wiki/Image:2000_Year_Temperature_Comparison.png ]
Figure 1-1 fits within this area of Figure 1-5
Figure 1-6 shows the temperature and CO2 information derived
from the Vostok ice cores. In January 1998, the collaborative ice-drilling project between
Russia, the United States, and France at the Russian Vostok station in East
Antarctica yielded the deepest ice core ever recovered, reaching a depth of
3,623 m. This shows the high correlation between CO2 and temperatures.
Figure 1-6: Temperature and CO2 For The Past
400,000 Years from Vostok Ice Cores
[From: UNEP/GRID-Arendal. Temperature and CO2
concentration in the atmosphere over the past 400 000 years.
UNEP/GRID-Arendal Maps and Graphics Library. 2000 http://maps.grida.no/go/graphic/temperature_and_co2_concentration_in_the_atmosphere_over_the_past_400_000_years.]
Figure 1-5 fits within this area of Figure 1-6
The IPCC, in the AR4 Scientific Basis report, Part 6 (May
2007), makes the following statement: “Variations
in CO2
over the last 420 kyr
broadly followed Antarctic temperature, typically by several centuries
to a millennium“
Figure 1-7 shows temperature reconstruction for five and a half million years constructed by combining measurements from 57 globally distributed deep-sea sediment cores (Lisiecki, L. E., and M. E. Raymo (2005), A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records, Paleoceanography). Labels at the upper left indicate regions where 100 kyr and 41 kyr cyclicity is observed. These periodicities match periodic changes in Earth's orbital. This image is from: [http://en.wikipedia.org/wiki/Image:Five_Myr_Climate_Change.png ]
Figure 1-7: Temperature For The Past 5.5 Million
Years from Sediment Cores
Figure 1-6 fits within this area of Figure 1-7
The historical record shows a general cooling trend over millions of years with the last million years exhibiting a distinct wide swing in temperature approximately every 100,000 years. Recent global averages are in a range similar to the temperatures of the Medieval Warm Period. Part 2 of this Global Warming Summary document discusses the Medieval Warm Period in greater detail.