Figure 1. Contributions to Earth's (positive) energy imbalance in 2005-2010. Estimates for the deep Southern and Abyssal Oceans are by Purkey and Johnson (2010) based on sparse observations. (Credit: NASA/GISS) + View larger image or PDF |
By James Hansen, Makiko Sato, Pushker Kharecha and
Karina von Schuckmann, Goddard Institute for Space Studies, NASA, January 2012
Deployment of an international array of Argo floats, measuring ocean
heat content to a depth of 2000 m, was completed during the past decade, allowing
the best assessment so far of Earth's energy imbalance. The observed planetary
energy gain during the recent strong solar minimum reveals that the solar
forcing of climate, although significant, is overwhelmed by a much larger net
human-made climate forcing. The measured imbalance confirms that, if other
climate forcings are fixed, atmospheric CO2 must be reduced to about
350 ppm or less to stop global warming. In our recently published paper (Hansen
et al., 2011), we also show that climate forcing by human-made aerosols (fine
particles in the air) is larger than usually assumed, implying an urgent need
for accurate global aerosol measurements to help interpret continuing climate
change.
Earth's energy imbalance is the difference between the amount of solar
energy absorbed by Earth and the amount of energy the planet radiates to space
as heat. If the imbalance is positive, more energy coming in than going out, we
can expect Earth to become warmer in the future — but cooler if the imbalance
is negative. Earth's energy imbalance is thus the single most crucial measure
of the status of Earth's climate and it defines expectations for future climate
change.
Energy imbalance arises because of changes of the climate forcings
acting on the planet in combination with the planet's thermal inertia. For
example, if the Sun becomes brighter, that is a positive forcing that will
cause warming. If Earth were like Mercury, a body composed of low conductivity
material and without oceans, its surface temperature would rise quickly to a
level at which the planet was again radiating as much heat energy to space as
the absorbed solar energy.
Earth's temperature does not adjust as fast as Mercury's due to the
ocean's thermal inertia, which is substantial because the ocean is mixed to
considerable depths by winds and convection. Thus it requires centuries for
Earth's surface temperature to respond fully to a climate forcing.
Climate forcings are imposed perturbations to Earth's energy balance.
Natural forcings include change of the Sun's brightness and volcanic eruptions
that deposit aerosols in the stratosphere, thus cooling Earth by reflecting
sunlight back to space. Principal human-made climate forcings are greenhouse
gases (mainly CO2), which cause warming by trapping Earth's heat
radiation, and human-made aerosols, which, like volcanic aerosols, reflect
sunlight and have a cooling effect.
Let's consider the effect of a long-lived climate forcing. Say the Sun
becomes brighter, staying brighter for a century or longer, or humans increase
long-lived greenhouse gases. Either forcing results in more energy coming in
than going out. As the planet warms in response to this imbalance, the heat
radiated to space by Earth increases. Eventually Earth will reach a global
temperature warm enough to radiate to space as much energy as it receives from
the Sun, thus stabilizing climate at the new level. At any time during this
process the remaining planetary energy imbalance allows us to estimate how much
global warming is still "in the pipeline."
Many nations began, about a decade ago, to deploy floats around the
world ocean that could "yo-yo" an instrument measuring ocean
temperature to a depth of 2 km. By 2006 there were about 3000 floats covering
most of the world ocean. These floats allowed von Schuckmann and Le Traon
(2011) to estimate that during the 6-year period 2005-2010 the upper 2 km of
the world ocean gained energy at a rate 0.41 W/m2 averaged over the
planet.
We used other measurements to estimate the energy going into the
deeper ocean, into the continents, and into melting of ice worldwide in the
period 2005-2010. We found a total Earth energy imbalance of +0.58±0.15 W/m2
divided as shown in Fig. 1.
The role of the Sun. The measured positive imbalance in 2005-2010 is
particularly important because it occurred during the deepest solar minimum in
the period of accurate solar monitoring (Fig. 2). If the Sun were the only
climate forcing or the dominant climate forcing, then the planet would gain
energy during the solar maxima, but lose energy during solar minima.
Figure
2. Solar irradiance in the era of accurate satellite data. Left scale is the
energy passing through an area perpendicular to Sun-Earth line. Averaged over
Earth's surface the absorbed solar energy is ~240 W/m2, so the
amplitude of solar variability is a forcing of ~0.25 W/m2. (Credit:
NASA/GISS)
+ View larger image or PDF
The fact that Earth gained energy at a rate 0.58 W/m2
during a deep prolonged solar minimum reveals that there is a strong positive
forcing overwhelming the negative forcing by below-average solar irradiance.
That result is not a surprise, given knowledge of other forcings, but it
provides unequivocal refutation of assertions that the Sun is the dominant climate
forcing.
Target CO2. The measured planetary energy imbalance
provides an immediate accurate assessment of how much atmospheric CO2
would need to be reduced to restore Earth's energy balance, which is the basic
requirement for stabilizing climate. If other climate forcings were unchanged,
increasing Earth's radiation to space by 0.5 W/m2 would require
reducing CO2 by ~30 ppm to 360 ppm. However, given that the
imbalance of 0.58±0.15 W/m2 was measured during a deep solar
minimum, it is probably necessary to increase radiation to space by closer to
0.75 W/m2, which would require reducing CO2 to ~345 ppm,
other forcings being unchanged. Thus the Earth's energy imbalance confirms an
earlier estimate on other grounds that CO2 must be reduced to about
350 ppm or less to stabilize climate (Hansen et al., 2008).
Aerosols. The measured planetary energy imbalance also allows us to
estimate the climate forcing caused by human-made atmospheric aerosols. This is
important because the aerosol forcing is believed to be large, but it is
practically unmeasured.
Figure
3. Schematic diagram of human-made climate forcings by greenhouse gases,
aerosols, and their net effect. (Credit: NASA/GISS)
+ View larger image or PDF
The human-made greenhouse gas (GHG) forcing is known to be about +3
W/m2 (Fig. 3). The net human-made aerosol forcing is negative
(cooling), but its magnitude is uncertain within a broad range (Fig. 3). The
aerosol forcing is complex because there are several aerosol types, with some
aerosols, such as black soot, partially absorbing incident sunlight, thus
heating the atmosphere. Also aerosols serve as condensation nuclei for water
vapor, thus causing additional aerosol climate forcing by altering cloud
properties. As a result, sophisticated global measurements are needed to define
the aerosol climate forcing, as discussed below.
The importance of knowing the aerosol forcing is shown by considering
the following two cases: (1) aerosol forcing about -1 W/m2, such
that the net climate forcing is ~ 2 W/m2, (2) aerosol forcing of -2
W/m2, yielding a net forcing ~1 W/m2. Both cases are
possible, because of the uncertainty in the aerosol forcing.
Which alternative is closer to the truth defines the terms of a
"Faustian bargain" that humanity has set for itself. Global warming
so far has been limited, as aerosol cooling has partially offset greenhouse gas
warming. But aerosols remain airborne only several days, so they must be pumped
into the air faster and faster to keep pace with increasing long-lived
greenhouse gases (much of the CO2 from fossil fuel emissions will
remain in the air for several millennia). However, concern about health effects
of particulate air pollution is likely to lead to eventual reduction of
human-made aerosols. Thereupon humanity's Faustian payment will come due.
If the true net forcing is +2 W/m2 (aerosol forcing -1 W/m2),
even a major effort to clean up aerosols, say reduction by half, increases the
net forcing only 25% (from 2 W/m2 to 2.5 W/m2). But if
the net forcing is +1 W/m2 (aerosol forcing -2 W/m2),
reducing aerosols by half doubles the net climate forcing (from 1 W/m2
to 2 W/m2). Given that global climate effects are already observed
(IPCC, 2007; Hansen et al., 2012), doubling the climate forcing suggests that
humanity may face a grievous Faustian payment.
Figure
4. Expected Earth energy imbalance for three choices of aerosol climate
forcing. Measured imbalance, close to 0.6 W/m2, implies that aerosol
forcing is close to -1.6 W/m2. (Credit: NASA/GISS)
+ View larger image or PDF
Most climate models contributing to the last assessment by the
Intergovernmental Panel on Climate Change (IPCC, 2007) employed aerosol
forcings in the range -0.5 to -1.1 W/m2 and achieved good agreement
with observed global warming over the past century, suggesting that the aerosol
forcing is only moderate. However, there is an ambiguity in the climate models.
Most of the models used in IPCC (2007) mix heat efficiently into the
intermediate and deep ocean, resulting in the need for a large climate forcing
(~2 W/m2) to warm Earth's surface by the observed 0.8°C over the
past century. But if the ocean mixes heat into the deeper ocean less
efficiently, the net climate forcing needed to match observed global warming is
smaller.
Earth's energy imbalance, if measured accurately, provides one way to
resolve this ambiguity. The case with rapid ocean mixing and small aerosol
forcing requires a large planetary energy imbalance to yield the observed
surface warming. The planetary energy imbalance required to yield the observed
warming for different choices of aerosol optical depth is shown in Fig. 4,
based on a simplified representation of global climate simulations (Hansen et
al., 2011).
Measured Earth energy imbalance, +0.58 W/m2 during
2005-2010, implies that the aerosol forcing is about -1.6 W/m2, a
greater negative forcing than employed in most IPCC models. We discuss multiple
lines of evidence that most climate models employed in these earlier studies
had moderately excessive ocean mixing, which could account for the fact that
they achieved a good fit to observed global temperature change with a smaller
aerosol forcing.
The large (negative) aerosol climate forcing makes it imperative that
we achieve a better understanding of the aerosols that cause this forcing.
Unfortunately, the first satellite capable of measuring detailed aerosol
physical properties, the Glory mission (Mishchenko et al., 2007), suffered a
launch failure. It is urgent that a replacement mission be carried out, as the
present net effect of changing emissions in developing and developed countries
is highly uncertain
Global measurements to assess the aerosol indirect climate forcing,
via aerosol effects on clouds, require simultaneous high precision polarimetric
measurements of reflected solar radiation and interferometric measurements of
emitted heat radiation with the two instruments looking at the same area at the
same time. Such a mission concept has been defined (Hansen et al., 1993) and
recent reassessments indicate that it could be achieved at a cost of about
$100M if carried out by the private sector without a requirement for undue
government review panels.
Related Link
NASA News Release: Earth's Energy Budget Remained Out of Balance Despite Unusually Low Solar
Activity
References
Hansen, J., W. Rossow, and I. Fung (Eds.), 1993: Long-term
Monitoring of Global Climate Forcings and Feedbacks, NASA Conf. Publ. 3234, Goddard Institute for
Space Studies, New York.
Hansen, J., Mki. Sato, P. Kharecha, D. Beerling, R. Berner, V.
Masson-Delmotte, M. Pagani, M. Raymo, D.L. Royer, and J.C. Zachos, 2008: Target
atmospheric CO2: Where should humanity aim? Open Atmos. Sci. J., 2, 217-231,
doi:10.2174/1874282300802010217.
Hansen, J., Mki. Sato, P. Kharecha, and K. von Schuckmann, 2011: Earth's energy
imbalance and implications.
Atmos. Chem. Phys., 11, 13421-13449, doi:10.5194/acp-11-13421-2011.
Hansen, J., Mki. Sato, and R. Ruedy, 2012: Perceptions of climate change: The new climate dice [WWW document], URL http://www.columbia.edu/~jeh1/mailings/2012/20120105_PerceptionsAndDice.pdf,
last accessed Jan. 6, 2012.
Intergovernmental Panel on Climate Change (IPCC), Climate Change 2007: The Physical Science Basis, S. Solomon, D. Qin, M. Manning, Z. Chen, M.
Marquis, K.B. Averyt, M. Tignor and H.L. Miller (Eds.), Cambridge Univ. Press,
996 pp.
Mishchenko, M.I., B. Cairns, G. Kopp, C.F. Schueler, B.A. Fafaul, J.E.
Hansen, R.J. Hooker, T. Itchkawich, H.B. Maring, and L.D. Travis, 2007: Accurate
monitoring of terrestrial aerosols and total solar irradiance: Introducing the
Glory mission.i Bull. Amer.
Meteorol. Soc., 88, 677-691, doi:10.1175/BAMS-88-5-677.
Purkey, S.G., and G.C. Johnson, 2010: Warming of global abyssal and
deep southern ocean between the 1990s and 2000s: contributions to global heat
and sea level rise budgets, J. Climate, 23, 6336-6351, doi:10.1175/2010JCLI3682.1.
Von Schuckmann, K., and P.-Y. Le Traon, 2011: How well can we derive
global ocean indicators from Argo data? Ocean Sci., 7, 783-791, doi:10.5194/os-7-783-2011.
Contact
No comments:
Post a Comment