User:William M. Connolley/Misc refs

Carbon cycle

Acceleration of global warming due to carbon-cycle feedbacks in a coupled climate model Peter M. Cox1, Richard A. Betts1, Chris D. Jones1, Steven A. Spall1 and Ian J. Totterdell2

The continued increase in the atmospheric concentration of carbon dioxide due to anthropogenic emissions is predicted to lead to significant changes in climate1. About half of the current emissions are being absorbed by the ocean and by land ecosystems2, but this absorption is sensitive to climate3, 4 as well as to atmospheric carbon dioxide concentrations5, creating a feedback loop. General circulation models have generally excluded the feedback between climate and the biosphere, using static vegetation distributions and CO2 concentrations from simple carbon-cycle models that do not include climate change6. Here we present results from a fully coupled, three-dimensional carbon–climate model, indicating that carbon-cycle feedbacks could significantly accelerate climate change over the twenty-first century. We find that under a 'business as usual' scenario, the terrestrial biosphere acts as an overall carbon sink until about 2050, but turns into a source thereafter. By 2100, the ocean uptake rate of 5 Gt C yr-1 is balanced by the terrestrial carbon source, and atmospheric CO2 concentrations are 250 p.p.m.v. higher in our fully coupled simulation than in uncoupled carbon models2, resulting in a global-mean warming of 5.5 K, as compared to 4 K without the carbon-cycle feedback.

[1]

Radiative and solar forcing

Philipona R, Durr B, Marty C, Ohmura A, Wild M (2004) Radiative Forcing - Measured At Earth ' S Surface - Corroborate The Increasing Greenhouse Effect. GEOPHYSICAL RESEARCH LETTERS 31 (3): art. no. L03202 FEB 6 2004

The Intergovernmental Panel of Climate Change (IPCC) confirmed concentrations of atmospheric greenhouse gases and radiative forcing to increase as a result of human activities. Nevertheless, changes in radiative forcing related to increasing greenhouse gas concentrations could not be experimentally detected at Earth's surface so far. Here we show that atmospheric longwave downward radiation significantly increased (+5.2(2.2) Wm(-2)) partly due to increased cloud amount (+1.0(2.8) Wm(-2)) over eight years of measurements at eight radiation stations distributed over the central Alps. Model calculations show the cloud-free longwave flux increase (+4.2(1.9) Wm(-2)) to be in due proportion with temperature (+0.82(0.41) degreesC) and absolute humidity (+0.21(0.10) g m(-3)) increases, but three times larger than expected from anthropogenic greenhouse gases. However, after subtracting for two thirds of temperature and humidity rises, the increase of cloud-free longwave downward radiation (+1.8(0.8) Wm(-2)) remains statistically significant and demonstrates radiative forcing due to an enhanced greenhouse effect.

J. L. Lean, Y.-M. Wang, and N. R. Sheeley. The effect of increasing solar activity on the Sun's total and open magnetic flux during multiple cycles: Implications for solar forcing of climate

We investigate the relationship between solar irradiance and cosmogenic isotope variations by simulating with a flux transport model the effect of solar activity on the Sun's total and open magnetic flux. As the total amount of magnetic flux deposited in successive cycles increases, the polar fields build up, producing a secular increase in the open flux that controls the interplanetary magnetic field which modulates the cosmic ray flux that produces cosmogenic isotopes. Non-axisymmetric fields at lower latitudes decay on time scales of less than a year; as a result the total magnetic flux at the solar surface, which controls the Sun's irradiance, lacks an upward trend during cycle minima. This suggests that secular increases in cosmogenic and geomagnetic proxies of solar activity may not necessarily imply equivalent secular trends in solar irradiance. Questions therefore arise about the interpretation of Sun-climate relationships, which typically assume that the proxies imply radiative forcing. [2]

(emphasis mine)

GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L03802, doi:10.1029/2004GL021167, 2005

E. Pallé: Possible satellite perspective effects on the reported correlations between solar activity and clouds

Recently some correlations between low cloud cover and solar activity have been reported in the literature. In this paper we show how the flux of GCR is found to correlate positively with the low clouds and negatively with higher clouds, supporting previous theoretical predictions linking atmospheric ionization by cosmic rays and cloud cover at different altitudes. All these correlations are however only marginally significant and the only strongly significant (negative) correlation is found between low and higher cloud layers. Thus, there is strong evidence that the solar-like variability in low cloud may be artificially induced by the satellite observing perspective.

GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L03802, doi:10.1029/2004GL021167, 2005

Direct heating

Block, A., K. Keuler and E. Schaller (2004), Impacts Of Anthropogenic Heat On Regional Climate Patterns, Geophys . Res . Lett . 31 ( 12 ): Art . No . L12211 Jun 26 2004

Four different simulations of a winter period in Central Europe are carried out to investigate the principle effect of anthropogenic heat release from the highly industrialized and populated Ruhrarea region ( Germany ) on regional climate conditions. The results reveal a permanent warming due to anthropogenic heat emissions over affected areas ranging from 0 .15 K over land area with an additional 2 W m(-2) anthropogenic heat flux up to 0.5 K over the Ruhrarea with additional 20 W m(-2) anthropogenic heat flux. The temperature effects induced by anthropogenic heat not only depend on the amount of added heat but also on orographical factors. No significant variations are found for precipitation.

UHI (ish)

Zhang, X.Y., M.A. Friedl, C.B. Schaaf, A.H. Strahler and A. Schneider (2004), The Footprint Of Urban Climates On Vegetation Phenology, Geophys . Res . Lett . 31 ( 12 ): Art . No . L12209 Jun 25 2004

Human activity, through changing land use and other activities, is the most fundamental source of environmental change on the Earth. Urbanization and the resultant "urban heat islands" provide a means for evaluating the effect of climate warming on vegetation phenology. Using data from the Moderate Resolution Imaging Spectroradiometer, we analyzed urban-rural differences in vegetation phenological transition dates and land surface temperatures for urban areas larger than 10 km(2) in eastern North America . The results show that the effect of urban climates on vegetation phenology decays exponentially with distance from urban areas with substantial influence up to 10 km beyond the edge of urban land cover, and that the ecological "footprint" of urban climates is about 2.4 times that of urban land use in eastern North America. The net effect is an increase in the growing season by about 15 days in urban areas relative to adjacent unaffected rural areas.

Long-term

Reconstructing Past Climate from Noisy Data (Hans von Storch, Eduardo Zorita, Julie Jones, Yegor Dimitriev, Fidel Gonzalez-Rouco, Simon Tett

http://www.sciencemag.org/cgi/content/abstract/1096109v1

Empirical reconstructions of the Northern Hemisphere (NH) temperature in the last millennium based on multy proxy records depict small-amplitude variations followed by a clear warming trend in the last two centuries. We use a coupled atmosphere-ocean model simulation of the last 1000 years as a surrogate climate to test the skill of these methods, particularly at multidecadal and centennial timescales. Idealized proxy records are represented by simulated grid-point temperature, degraded with statistical noise. The centennial variability of the NH temperature is underestimated by the regression-based methods applied here, suggesting that past variations may have been at least a factor of two larger than indicated by empirical reconstructions.

Model predictions of spatial var of cl ch, inc polar amplification

M. M. Holland and C. M. Bitz, Polar amplification of climate change in coupled models, Climate Dynamics, 2003 (21:221-232) [3]

nb: this is a quote from the intro, not the abstract:

Climate model simulations have shown that ice albedo feedbacks associated with variations in snow and sea-ice coverage are a key factor in positive feedback mechanisms which amplify climate change at high northern latitudes (e.g., Manabe and Stouffer 1980). In addition, variations in the thickness of sea-ice tend to reinforce surface atmospheric temperature anomalies by altering the heat and moisture transfer from the ocean to the atmosphere. Clouds respond to tropospheric temperature variations and most studies agree that clouds yield net positive radiative forcing at the surface in high latitudes (e.g., Curry et al. 1996). Hence, an increase in high-latititude cloud cover may also contribute to Northern Hemisphere polar amplification. High-latitude climate sensitivity also depends on changes in heat transported by the atmosphere and/or ocean.

There is agreement among models that the Arctic warms more than subpolar regions when subject to increasing levels of greenhouse gases (GHG) in the atmosphere. In contrast, high southern latitudes exhibit a minimum warming in coupled simulations due to changes in ocean heat uptake (IPCC 2001). At high latitudes in both hemispheres the range of warming across global climate models is considerable, with the range of warming in the Arctic being larger than elsewhere on Earth (IPCC 2001).

Misc

A. Tabazadeh, Y. S. Djikaev and H. Reiss: Surface crystallization of supercooled water in clouds, PNAS | December 10, 2002 | vol. 99 | no. 25 | 15873-15878 [4]

The process by which liquid cloud droplets homogeneously crystallize into ice is still not well understood. The ice nucleation process based on the standard and classical theory of homogeneous freezing initiates within the interior volume of a cloud droplet. Current experimental data on homogeneous freezing rates of ice in droplets of supercooled water, both in air and emulsion oil samples, show considerable scatter. For example, at –33°C, the reported volume-based freezing rates of ice in supercooled water vary by as many as 5 orders of magnitude, which is well outside the range of measurement uncertainties. Here, we show that the process of ice nucleus formation at the air (or oil)-liquid water interface may help to explain why experimental results on ice nucleation rates yield different results in different ambient phases. Our results also suggest that surface crystallization of ice in cloud droplets can explain why low amounts of supercooled water have been observed in the atmosphere near –40°C.