Editor's CommentThe International Panel on Climate Change (IPCC)'s Fifth Assessment Report (AR5) presents new evidence of climate change based on observations of the climate system, paleoclimate archives, and theoretical studies of climate processes. (It also includes the results of modeling and simulations which are not discussed here.) We present and compare observations on land and sea surface temperatures and their historical rate of change from AR5. AR5 reported that the latest combined sea and land surface temperatures which were computed independently by four different groups were found to be substantially in agreement. We also report the latest observations of greenhouse gas concentrations and their historical rate of change from AR5.


This article considers three important factors relating to climate change.

  1. Energy budget and the greenhouse effect
  2. Temperature
  3. Greenhouse gas concentrations

1. Energy budget and the greenhouse effect


The energy budget of the Earth is a fundamental concept that underlies the IPCC's assessment reports. The energy budget is a conceptual framework for understanding the energy fluxes that are responsible for heating and cooling the Earth. From the persective of the IPCC the priamry objective of the energy budget is to estimate the net energy imbalance between the solar radiation absorbed by the Earth and the thermal radiation lost by the Earth to outer space. This is a challenge because the imbalance is a small difference on the order of one watt per square meter between large numbers on the order of hundreds of watts per square meter.

There are two important sources of energy, the sun and the Earth itself.

The sun radiates energy at many wavelengths from the ultraviolet to the visible that impinges on the Earth's atmosphere. Some of this is absorbed by the Earth's atmosphere and surface and some of it is reflected back to outer space.

The Earth itself also radiates energy, but in the form of heat at infrared wavelengths. Some of this is absorbed by the atmosphere and some of it is radiated to outer space.

Energy budget current conditions IPCC AR5
Energy budget current conditions IPCC AR5


The energy budget of the Earth is broken down into contributions from different types of energy flows.

  • Incoming solar TOA (at the top of the atmosphere) = average solar radiation impinging on top of Earth's atmosphere
  • Solar reflected TOA = solar radiation reflected by Earth's atmosphere
  • Solar down surface = solar radiation hitting Earth's surface
  • Solar absorption surface = solar radiation absorbed by Earth's surface
  • Solar reflected surface = solar radiation reflected by Earth's surface
  • Thermal up surface = heat radiated by Earth's surface to atmosphere
  • Sensible heat = heat exchanged between Earth's surface and atmosphere due to convection
  • Thermal outgoing TOA = heat radiated from Earth'a atmosphere to space
  • Greenhouse gas effect = back radiation to the surface from heat retained on Earth's surface by greenhouse gases (CO2, CH4, N2O)
  • Evaporation = heat conveyed from Earth's surface to atmosphere by evaporation of water

The major atmospheric gases (oxygen and nitrogen) are transparent to incoming sunlight, and are also transparent to outgoing thermal infrared. However, water vapor, carbon dioxide, methane, and other trace gases are opaque to many wavelengths of thermal infrared energy. The Earth's surface radiates the net equivalent of 17 percent of incoming solar energy as thermal infrared, whereas the amount that directly escapes to space is only about 12 percent of incoming solar energy. The remaining fraction—a net 5-6 percent of incoming solar energy—is transferred to the atmosphere when greenhouse gas molecules absorb thermal infrared energy radiated by the surface.

Because greenhouse gas molecules radiate heat in all directions, some of it spreads downward and ultimately comes back into contact with the Earth’s surface, where it is absorbed. The temperature of the surface becomes warmer than it would be if it were heated only by direct solar heating. This supplemental heating of the Earth’s surface by the atmosphere is the natural greenhouse effect.

2. Changes in Temperature


Land-Surface Air Temperature (LSAT)

Global Land-Surface Air Temperature (LSAT) has been increasing over the instrumental period record with the warming rate approximately double that reported over the oceans since 1979. Substantial developments have occurred in more modern and accurate data records. These new developments have improved understanding of data issues and uncertainties, allowing better quantification of regional changes. This reinforces confidence in the reported globally averaged LSAT time series behaviour.

The following graph (Figure 2.14) compares four sets of data records from different institutions. The average values of temperature trends from 1880 to 2012 is presented in Table 2.14 below.

Goddard Institute of Space Studies (GISS) accounts for urban impacts through night lights adjustments. CRUTEM4 incorporates additional station series and also newly homogenized versions of many individual station records. Berkeley uses a method that is substantially distinct from the others.

Despite the range of approaches, the long-term variations and trends broadly agree among these various LSAT estimates, particularly after 1900.

Image

In general the temperature increases are consistent over the period of observation. The average trends of increase are given in Table 2.4. As shown in the table, from 1880 until 2012 the trend of temperature increase of LSAT varies between 0.086 degrees °C per decade (from CRUTEMA) to 0.095 degrees °C per decade (from GISS).

Table 2.4 : Trends in LSAT global average values (°C per decade)

DatasetYear 1880 - 2012
CRUTEMA0.086 +/- 0.015
GHCNv3.2.00.094 +/- 0.016
GISS0.095 +/- 0.015
Berkeley0.094 +/- 0.013

.
Sea Surface Temperature (SST) and Marine Air Temperature

Sea Surface Temperature (SST) has been observed since 1880. Based on these observations there is strong evidence that SST is increasing. Table 2.5 below shows two different data sources for SST which in general are in close agreement.

Table 2.5 Trend in SST (°C per decade)

Dataset Year 1880 - 2012
HadSST3 (Kennedy et al, 2011)0.054 +/- 0.012
HadSST2 (Rayner et al, 2006)0.051 +/- 0.015


In the early days, most SST observations were obtained from moving ships. Buoy measurements comprise a significant and increasing fraction of in situ SST measurements from the 1980s onward. Improvements in the understanding of uncertainty have been expedited by the use of metadata and the recovery of observer instructions and other related documents. The adjustments, made using ship observations of Night Marine Air Temperature data (NMAT) and other sources, had a striking effect on the SST global mean estimates. These observation results from different methods have been compared. As can be seen in Fig 2.15 below, observations from these different methods are in a good agreement.

Image
Fig. 2.15 a) Fractional contributions of observations made by different measurement methods:
(blue) bucket observation
(green) Engine room intake (ERI) and hull contact sensor observations
(red) moored and drifting buoys
(yellow)others
Fig. 2.15 b) Global annual SST anomalies based on different kinds of data:
(green) ERI and hull contact sensor
(blue) bucket
(red) buoy
(black) all

The report states that it is certain that SSTs have increased since the beginning of the 20th century. Major improvements in availability of metadata and data completeness have been made, and a number of new global SST records have been produced. Comparison of new SST data records obtained by different measurement methods, including satellite data, have resulted in better understanding of uncertainties and biases in the records. While these innovations have helped highlight and quantify uncertainties, they do not alter the conclusion that global SSTs have increased both since the 1950s and since the late 19th century.

Global Combined Land and Sea Surface Temperature (GMST)

It can be seen from Table 2.7 that three different observations have shown that the average rate of temperature increase lies between 0.062 to 0.065 °C per decade.

Table 2.7 : Trends in GMST (°C per decade)

DatasetYear 1880 - 2012
HadCRUT40.062 +/- 0.012
NCDC MLOST0.064 +/- 0.015
GISS0.065 +/- 0.015


The report states that it is certain that the GMST has increased. In the last 50 years the rate of increase is almost double that of the last 100 years. Subsequent developments in LSAT and SST have led to better understanding of the data and its uncertainties.

Image
Figure 2.19: Decadal GMST anomalies (white vertical lines in grey blocks) and their uncertainties (90% confidence intervals as grey blocks) based upon the combined Land-Surface Air Temperature (LSAT) and Sea Surface Temperature (SST). Anomalies are relative to 1961–1990. 1850s indicates the period 1850-1859 etc.

Starting in the 1980s each decade has been significantly warmer than all preceding decades since the 1850s. All ten of the warmest years have occurred since 1997, with 2010 and 2005 tied for the warmest year on record in all three data sets. However, uncertainties in individual annual values are sufficiently large that the ten warmest years are statistically indistinguishable from one another.

In summary, the report states that it is certain that globally averaged surface temperatures have increased since the late 19th century. Each of the past three decades has been warmer than all the previous decades in the instrumental record, and the decade of the 2000s has been the warmest. The global combined land and ocean temperature data (GMST) show an increase of about 0.89°C (0.69°C–1.08°C) over the period 1901–2012 and about 0.72°C (0.49°C–0.89°C) over the period 1951–2012 assuming a linear trend. Despite the fact that over decades the warming trend is robust, there exists substantial variability between years in the rate of warming with several periods exhibiting almost no linear trend.

3. Changes in Atmospheric Composition of Greenhouse Gases


In 2007, AR4 concluded that increasing atmospheric concentrations of greenhouse gases (GHG) resulted in a 9% increase in their radiative forcing (contribution to warming the Earth's surface) from 1998 to 2005. Since 2005, the atmospheric abundances of many GHG increased further, but the concentrations of some ozone depleting substances (ODS) whose production and use were controlled by the Montreal Protocol on Substances that Deplete the Ozone Layer decreased.

Based on updated in situ observations, AR5 concludes that these trends resulted in a 7.5% increase in radiative forcing from greenhouse gases from 2005 to 2011, with CO2 contributing 80%. Of note is an increase in the average growth rate of atmospheric CH4 from ~0.5 ppb/year during 1999–2006 to ~6 ppb/year from 2007 through 2011.

Table 2.1
The following data derived from Table 2.1 summarizes globally, annually averaged GHG mole fractions from four independent measurement programs. Sampling strategies and techniques for estimating global means and their uncertainties vary among programs.

Global annual mean surface dry-air mole fractions and their change since 2005 for well-mixed GHG
SpeciesLifetime (yr)Radiative Efficiency (RE)2011 Global Annual MeanGlobal Increase 2005-2011
CO2 (ppm) 1.37 x10-5390.48 +/-0.2811.67 +/-0.37
CH4 (ppb)9.13.63 x10-41803.1 +/-4.828.9 +/-6.8
N2O (ppb)1313.03 x10-3324.0 +/-0.1 4.7 +/-0.2
SF632000.575 7.26 +/-0.02 1.65 +/-0.03
CF4500000.1 79.0 +/-0.1 4.0 +/-0.2
C2F6100000.26 4.16 +/-0.02 0.50 +/-0.03
HFC-12528.20.2199.58 +/-0.045.89 +/-0.07
HFC-134a13.40.15962.4 +/-0.328.2 +/-0.4
CFC-11450.263236.9 +/-0.1-12.7 +/-0.2
CFC-121000.32529.5 +/-0.2-13.4 +/-0.3
HCFC-2211.90.2213.4 +/-0.844.6 +/-1.1
*Source: Table 2.1 (p301) AR5, Final Draft Underlying Scientific-Technical Assessment
*Four measurement networks are used for Table 2.1 in Chapter 2, AR5. The most complete set is from AGAGE = Advanced Global Atmospheric Gases Experiment. The other network measurements (see Table 2.1 and notes) are consistent with the above results from AGAGE.
*Units are parts per trillion (ppt) except where noted. Ppb = parts per billion. Ppm = parts per million.
*Uncertainties are 90% confidence intervals.
*Data is also included in Table 2.1 for other Species.
*Detailed data for radiative efficiencies (RE) and lifetime/adjustment times is provided in Table 8.A.1 in Chapter 8 (pp 8-88 to 8-99), AR5.
*Lifetime indicates how long the gas remains in the atmosphere.
*Radiative efficiency describes how effectively a molecule can affect climate. It is multiplied by its atmospheric concentration to determine the total climate impact.


Source ; IPCC AR5 Chapter 2