James Hansen | Columbia University (original) (raw)

Papers by James Hansen

A common view is that the current global warming rate will continue or accelerate. But we argue t... more A common view is that the current global warming rate will continue or accelerate. But we argue that rapid warming in recent decades has been driven by non-CO2 greenhouse gases (GHGs), such as CFCs, CH4 and N20, not by the products of fossil fuel burning, CO2 and aerosols, whose positive and negative climate forcings are partially offsetting. The growth rate of non-CO2 GHGs has declined in the past decade. If sources of CH4 and 03 precursors were reduced in the future, the change of climate forcing by non-CO2 GHGs in the next 50 years could be near zero. Combined with a reduction of black carbon emissions and

Nature Reviews Earth & Environment

Aerosols are small liquid or solid particles suspended in the atmosphere 1. They can be emitted d... more Aerosols are small liquid or solid particles suspended in the atmosphere 1. They can be emitted directly (such as dust, sea salt, black carbon (BC) and volcanic aerosols) or formed indirectly through chemical reactions (including sulfate, nitrate, ammonium and secondary organic aerosols). Owing to their relatively short lifetime, aerosol concentrations typically peak near their sources. Desert regions (such as North Africa and the Middle East), industrial regions (such as East and South Asia) and biomass-burning regions (such as South America and South Africa) are, therefore, characterized by high mass concentrations (Fig. 1). Aerosols exhibit complicated compositions and vary substantially in shape and size, typically ranging between 0.01 and 10 μm (reF. 2). Depending on these structural and compositional characteristics, aerosols can scatter and/or absorb shortwave radiation, as quantified through the single-scattering albedo (SSA; Table 1). Purely scattering aerosols include sulfates, nitrates, ammonium and sea-salt particles, whereas absorbing aerosols are primarily BC, with dust and organic carbon partly absorbing in the ultraviolet (UV) spectrum 3. Aerosols have a direct bearing on Earth's energy balance and, therefore, on climate. For instance, aerosol scattering and absorption alters the radiation balance and atmospheric stability through perturbations to the vertical temperature profile. Aerosols can further serve as cloud condensation nuclei (CCN) or ice-nucleating particles (INPs), which modify the reflectivity and lifetime of clouds through microphysical processes. Collectively, these influences are quantified as aerosol forcing: the change of net radiative flux at a specified level of the atmosphere, often assessed relative to estimated pre-industrial conditions 4. Globally, anthropogenic aerosols are estimated to produce a net cooling ~−1.3 ± 0.7 W m −2 at the top of the atmosphere; −0.3 ± 0.3 W m −2 is attributed to the aerosol-radiation interaction (ARI), −1.0 ± 0.7 W m −2 to aerosol-cloud interactions, ~−1.15 W m −2 to total forcing from scattering aerosols and ~+0.12 W m −2 to BC 4. This combined aerosol forcing offsets roughly one-third of the warming from anthropogenic greenhouse gases (GHGs). However, the large spread in the estimated aerosol forcing leads to large discrepancies in climate sensitivity 5,6. Thus, aerosols are considered to be the largest contributor of uncertainty in quantifying present-day climate change 4. Much of this uncertainty in aerosol forcing arises from both the lack of separate global constraints on aerosol optical and microphysical properties (optical depth, size distribution, hygroscopicity and mixing state, among others) and the inaccurate representation of them in climate models 7-10. In particular, aerosol SSA is further

state and declare as follows: 1. I am over 18 years of age and am competent to testify regarding ... more state and declare as follows: 1. I am over 18 years of age and am competent to testify regarding the following. 2. I submit this report as my personal opinion as a private citizen.

Nature Climate Change, 2016

Journal of Geophysical Research: Atmospheres, 2001

Several previous studies have attempted to remove the effects of explosive volcanic eruptions and... more Several previous studies have attempted to remove the effects of explosive volcanic eruptions and El Niño‐Southern Oscillation (ENSO) variability from time series of globally averaged surface and tropospheric temperatures. Such work has largely ignored the nonzero correlation between volcanic signals and ENSO. Here we account for this collinearity using an iterative procedure. We remove estimated volcano and ENSO signals from the observed global mean temperature data, and then calculate trends over 1979–1999 in the residuals. Residual trends are sensitive to the choice of index used for removing ENSO effects and to uncertainties in key volcanic parameters. Despite these sensitivities, residual surface and lower tropospheric (2LT) trends are almost always larger than trends in the raw observational data. After removal of volcano and ENSO effects, the differential warming between the surface and lower troposphere is generally reduced. These results suggest that the net effect of volca...

Global Biogeochemical Cycles, 2008

Unconstrained CO 2 emission from fossil fuel burning has been the dominant cause of observed anth... more Unconstrained CO 2 emission from fossil fuel burning has been the dominant cause of observed anthropogenic global warming. The amounts of ''proven'' and potential fossil fuel reserves are uncertain and debated. Regardless of the true values, society has flexibility in the degree to which it chooses to exploit these reserves, especially unconventional fossil fuels and those located in extreme or pristine environments. If conventional oil production peaks within the next few decades, it may have a large effect on future atmospheric CO 2 and climate change, depending upon subsequent energy choices. Assuming that proven oil and gas reserves do not greatly exceed estimates of the Energy Information Administration, and recent trends are toward lower estimates, we show that it is feasible to keep atmospheric CO 2 from exceeding about 450 ppm by 2100, provided that emissions from coal, unconventional fossil fuels, and land use are constrained. Coal-fired power plants without sequestration must be phased out before midcentury to achieve this CO 2 limit. It is also important to ''stretch'' conventional oil reserves via energy conservation and efficiency, thus averting strong pressures to extract liquid fuels from coal or unconventional fossil fuels while clean technologies are being developed for the era ''beyond fossil fuels''. We argue that a rising price on carbon emissions is needed to discourage conversion of the vast fossil resources into usable reserves, and to keep CO 2 beneath the 450 ppm ceiling.

Geophysical Research Letters, 2003

Anthropogenic emissions of fine black carbon (BC) particles, the principal light‐absorbing atmosp... more Anthropogenic emissions of fine black carbon (BC) particles, the principal light‐absorbing atmospheric aerosol, have varied during the past century in response to changes of fossil‐fuel utilization, technology developments, and emission controls. We estimate historical trends of fossil‐fuel BC emissions in six regions that represent about two‐thirds of present day emissions and extrapolate these to global emissions from 1875 onward. Qualitative features in these trends show rapid increase in the latter part of the 1800s, the leveling off in the first half of the 1900s, and the re‐acceleration in the past 50 years as China and India developed. We find that historical changes of fuel utilization have caused large temporal change in aerosol absorption, and thus substantial change of aerosol single scatter albedo in some regions, which suggests that BC may have contributed to global temperature changes in the past century. This implies that the BC history needs to be represented realist...

Climatic Change, 2005

In a recent article (Hansen, 2004) I included a photograph taken by Roger Braithwaite with a rush... more In a recent article (Hansen, 2004) I included a photograph taken by Roger Braithwaite with a rushing stream pouring into a hole in the Greenland ice sheet. The photo relates to my contention that disintegration of ice sheets is a wet, potentially rapid, process, and consequent sea level rise sets a low limit on the global warming that can be tolerated without risking dangerous anthropogenic interference with climate. I asked glaciologist Jay Zwally if I would be crucified for a caption such as: "On a slippery slope to Hell, a stream of snowmelt cascades down a moulin on the Greenland ice sheet. The moulin, a near-vertical shaft worn in the ice by surface water, carries water to the base of the ice sheet. There the water is a lubricating fluid that speeds motion and disintegration of the ice sheet. Ice sheet growth is a slow dry process, inherently limited by the snowfall rate, but disintegration is a wet process, spurred by positive feedbacks, and once well underway it can be explosively rapid." Zwally replied "Well, you have been crucified before, and March is the right time of year for that, but I would delete 'to Hell' and 'explosively"'. I thought immediately of the fellow who went over Niagara Falls without a barrel. Would not he consider that a joy ride, compared to slipping on the banks of the rushing melt-water stream, clawing desperately in the freezing water before being hurtled down the moulin more than a kilometer, and eventually being crushed by the giant grinding glacier? "A slippery slope to Hell" did not seem like an exaggeration. On the other hand, I was using "slippery slope" mainly as a metaphor for the danger posed by global warming. So I changed "Hell" to "disaster." What about "explosively"? Consider the situation during past ice sheet disintegrations. In melt-water pulse 1A, about 14,000 years ago, sea level rose about 20 m in approximately 400 years (Kienast et al., 2003). That is an average of 1 m of sea level rise every 20 years. The nature of glacier disintegration required for delivery of that much water from the ice sheets to the ocean would be spectacular (5 cm of sea level, the mean annual change, is about 15,000 cubic kilometers of water). "Explosively" would be an apt description, if future ice sheet disintegration were to occur at a substantial fraction of the melt-water pulse 1A rate. Are we on a slippery slope now? Can human-made global warming cause ice sheet melting measured in meters of sea level rise, not centimeters, and can this occur in centuries, not millennia? Can the very inertia of the ice sheets, which protects us from rapid sea level change now, become our bête noire as portions of

Atmosphere-Ocean, 2007

Evidence based on numerical simulations is presented for a strong correlation between the North A... more Evidence based on numerical simulations is presented for a strong correlation between the North Atlantic Oscillation (NAO) and the North Atlantic overturning circulation. Using an ensemble of numerical experiments with a coupled ocean-atmosphere model including both natural and anthropogenic forcings, it is shown that the weakening of the thermohaline circulation (THC) could be delayed in response to a sustained upward trend in the NAO, which was observed over the last three decades of the twentieth century, 1970-99. Overall warming and enhanced horizontal transports of heat from the tropics to the subpolar North Atlantic overwhelm the NAO-induced cooling of the upper ocean layers due to enhanced fluxes of latent and sensible heat, so that the net effect of warmed surface ocean temperatures acts to increase the vertical stability of the ocean column. However, the strong westerly winds cause increased evaporation from the ocean surface, which leads to a reduced fresh water flux over the western part of the North Atlantic. Horizontal poleward transport of salinity anomalies from the tropical Atlantic is the major contributor to the increasing salinities in the sinking regions of the North Atlantic. The effect of positive salinity anomalies on surface ocean density overrides the opposing effect of enhanced warming of the ocean surface, which causes an increase in surface density in the Labrador Sea and in the ocean area south of Greenland. The increased density of the upper ocean layer leads to deeper convection in the Labrador Sea and in the western North Atlantic. With a lag of four years, the meridional overturning circulation of the North Atlantic shows strengthening as it adjusts to positive density anomalies and enhanced vertical mixing. During the positive NAO trend, the salinity-driven density instability in the upper ocean, due to both increased northward ocean transports of salinity and decreased atmospheric freshwater fluxes, results in a strengthening overturning circulation in the North Atlantic when the surface atmospheric temperature increases by 0.3°C and the ocean surface temperature warms by 0.5°to 1°C. RÉSUMÉ [Traduit par la rédaction] Nous présentons des preuves basées sur des simulations numériques d'une forte corrélation entre l'oscillation nord-atlantique (ONA) et la circulation de renversement dans l'Atlantique Nord. Au moyen d'un ensemble d'expériences numériques avec un modèle couplé océan-atmosphère qui inclut à la fois les forçages naturel et anthropique, nous montrons que l'affaiblissement de la circulation thermohaline pourrait être retardé par suite d'une tendance à la hausse soutenue dans l'ONA, tendance qui a été observée au cours des trois dernières décennies du vingtième siècle, soit la période 1970-1999. Le réchauffement général et l'augmentation des transports horizontaux de chaleur des tropiques vers l'Atlantique Nord subpolaire font plus que compenser le refroidissement causé par l'ONA des couches supérieures de l'océan à cause de l'augmentation des flux de chaleur latente et sensible, de sorte que l'effet net des températures plus élevées de la surface de l'océan fait augmenter la stabilité verticale de la colonne océanique. Cependant, les forts vents d'ouest entraînent une évaporation accrue à la surface de l'océan, ce qui mène à un flux d'eau douce réduit dans la partie ouest de l'Atlantique Nord. Le transport horizontal vers le pôle des anomalies de salinité à partir de l'Atlantique tropical est le principal contributeur de l'accroissement de salinité dans les régions de plongée d'eau de l'Atlantique Nord. L'effet des anomalies positives de salinité sur la densité de la surface océanique annule l'effet opposé du réchauffement accru de la surface océanique, ce qui entraîne un accroissement de la densité à la surface dans la mer du Labrador et dans la région de l'océan au sud du Groenland. La densité accrue des couches supérieures de l'océan occasionne une convection plus profonde dans la mer du Labrador et dans l'ouest de l'Atlantique Nord. Avec un retard de quatre ans, la circulation de renversement méridienne dans l'Atlantique Nord se renforce en réponse aux anomalies de densité positives et au mélange vertical accru.

Previous research has identified links between changes in sea surface temperature (SST) and hurri... more Previous research has identified links between changes in sea surface temperature (SST) and hurricane intensity. We use climate models to study the possible causes of SST changes in Atlantic and Pacific tropical cyclogenesis regions. The observed SST increases in these regions range from 0.32 to 0.67 • C over the 20th century. The 22 climate models examined here suggest that century-timescale SST changes of this magnitude cannot be explained solely by unforced variability of the climate system, even under conservative assumptions regarding the magnitude of this variability. Model simulations that include external forcing by combined anthropogenic and natural factors are generally capable of replicating observed SST changes in both tropical cyclogenesis regions.

Recent analyses indicate that the amount of atmospheric CO 2 required to cause dangerous climate ... more Recent analyses indicate that the amount of atmospheric CO 2 required to cause dangerous climate change is at most 450 ppm, and likely less than that. Reductions of non-CO 2 climate forcings can provide only moderate, albeit important, adjustments to the CO 2 limit. Realization of how close the planet is to 'tipping points' with unacceptable consequences, especially ice sheet disintegration with sea level rise out of humanity's control, has a bright side. It implies an imperative: we must find a way to keep the CO 2 amount so low that it will also avert other detrimental effects that had begun to seem inevitable, e.g., ocean acidification, loss of most alpine glaciers and thus the water supply for millions of people, and shifting of climatic zones with consequent extermination of species. Here I outline from a scientific perspective actions needed to achieve low limits on CO 2 and global warming. These changes are technically feasible and have ancillary benefits. Achievement of needed changes requires overcoming the spurious argument that developed and developing countries have equivalent responsibilities, as well as overcoming special interests advocating minimalist or counterproductive actions such as corn-based ethanol and liquid-fuelfrom-coal programs. This paper consists of written testimony that I delivered as a private citizen to the Select Committee on Energy Independence and Global Warming, United States House of Representatives on 26 April 2007. I have added to that testimony: this abstract, references for several statements in the testimony, and some specificity in the final section on solutions.

metric measurements is a challenging research task, analyses should eventually become routine eno... more metric measurements is a challenging research task, analyses should eventually become routine enough to be continued as part of the NOAA operational observing system, as was the case for CO 2 monitoring. Other climate measurements. Climsat is not intended to provide all missing climate forcings. Measurements of changes in solar irradiance, which could be done from another small satellite, are also required. Several parameters monitored by Climsat, in particular properties of fine particles in the lower atmosphere, the vertical profile of ozone, and cloud microphysical properties, are very difficult to measure accurately, so that successful monitoring also will require long-term observations from selected ground sites and field campaigns for comparison of the satellite results with measurements made on or near the Earth's surface. Another important research objective is diagnosis of the key heat and water transport processes that affect the sensitivity of climate. We particularly note the need to observe precipitation, ocean energy transports, and atmospheric heat and water vapor transports. Climsat generally will not address these difficult measurements. Experimental programs, similar to the Tropical Rainfall Measuring Mission, the World Ocean Circulation Experiment, and Acoustic Thermometry of Ocean Climate, must be continued and expanded upon to develop these capabilities. Operational observations of proven high accuracy, such as microwave temperature measurements, should be continued. Climsat status. Congress allocated 515M in 1991 to initiate Climsat on the basis of an article published in Issues in Science and Technology summarizing the proposed project. However, the Administration declined to undertake the project and the funding was later rescinded by Congress. Thus Climsat is currently a pre-project study without funding. We will continue discussions with the scientific community in the coming months and welcome any comments. We intend to request approval for a Climsat project in 1993. If it is approved, we anticipate a "Dear Colleague" letter or "Announcement of Opportunity" to solicit participation. Relation to EOS. The Earth Observing System and Climsat are complementary, not competitive. EOS is well suited for detailed measurements which will be crucial for studying many global change processes, including some climate processes. But EQS does not measure all climate forcings and is not well suited for long-term monitoring. Because of its high cost and the absence of "hot spares" to replace a failed instrument or spacecraft, EOS is not likely to provide continuous multidecadal monitoring. EQS does not provide the required space-time sampling and coverage as Climsat, which has two identical satellites. The Climsat approach also allows instrument cross-calibration when one must be replaced, which is critical to long-term data precision. EOS does not adequately sample diurnal variations, which are particularly important in defining cloud forcings and feedbacks. EQS does not plan to address the greatest uncertainty in human-made climate forcings, lower atmospheric fine particles, with the required precision until the second AM spacecraft in 2003. Although there is a proposal to include two Climsat instruments on later EQS spacecraft, that plan is not budgeted, and thus of questionable realism; even if carried out, it would not satisfy the need for long-term homogeneous data with global coverage and diurnal sampling, and even this inadequate data would not be obtained until another decade or more. Data availability. The Climsat data system will be configured to make calibrated measurements and data products widely available as they are produced, without a period of proprietary use by project participants. Educational outreach. Climsat data and data products are well suited to contribute to teaching Earth sciences in schools because they will provide a low volume, comprehensive, ongoing description of important climate parameters. Global maps of monthly-averaged distributions of surface and atmospheric properties will reveal seasonal and interannual changes which can be used to illustrate global change topics to students. Beginning researchers can analyze differences of intensity or polarization among wavelengths for particular regions or problems. These data volumes can be readily distributed over computer networks available in the mid-1990s. We intend to propose a liason with educational organizations for this purpose as part of the Climsat program.

J. Hansen, M. Sato, R. Ruedy, P. Kharecha, A. Lacis, R. Miller, L. Nazarenko, K. Lo, G. A. Schmid... more J. Hansen, M. Sato, R. Ruedy, P. Kharecha, A. Lacis, R. Miller, L. Nazarenko, K. Lo, G. A. Schmidt, G. Russell, I. Aleinov, S. Bauer, E. Baum, B. Cairns, V. Canuto, M. Chandler, Y. Cheng, A. Cohen, A. Del Genio, G. Faluvegi, E. Fleming, A. Friend, T. Hall, C. Jackman, J. Jonas, M. Kelley, N. Y. Kiang, D. Koch, G. Labow, J. Lerner, S. Menon, T. Novakov, V. Oinas, Ja. Perlwitz, Ju. Perlwitz, D. Rind, A. Romanou, R. Schmunk, D. Shindell, P. Stone, S. Sun, D. Streets, N. Tausnev, D. Thresher, N. Unger, M. Yao, and S. Zhang NASA Goddard Institute for Space Studies, New York, New York, USA Columbia University Earth Institute, New York, New York, USA Sigma Space Partners LLC, New York, New York, USA Department of Earth and Environmental Sciences, Columbia University, New York, New York, USA Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, USA Clean Air Task Force, Boston, Massachusetts, USA NASA Goddard Space Flight Center, Greenbelt, Maryland...

I have been told by a high government official that I should not talk about “dangerous anthropoge... more I have been told by a high government official that I should not talk about “dangerous anthropogenic interference” with climate, because we do not know how much humans are changing the Earth’s climate or how much change is “dangerous”. Actually, we know quite a lot. Natural regional climate fluctuations remain larger today than human-made effects such as global warming. But data show that we are at a point where human effects are competing with nature and the balance is shifting. Ominously, the data show that human effects have been minimized by a Faustian bargain: global warming effects have been mitigated by air pollutants that reduce the amount of sunlight reaching the Earth’s surface. This Faustian bargain has a time limit, and the payment is now coming due. Actions that would alleviate human distortions of nature are not only feasible but make sense for other reasons, including our economic well-being and national security. However, our present plan in the United States is to w...

Figure 1. Wind and tides mix the ocean to great depths. Thus, because of the thermal inertia of t... more Figure 1. Wind and tides mix the ocean to great depths. Thus, because of the thermal inertia of this ocean water, it requires at least several decades for the ocean temperature to respond fully to a climate forcing. All glaciers in Glacier National Park are retreating inexorably to their final demise. Global warming is real, and the melting ice is an apt portent of potentially disastrous consequences. Yet most gloom-and-doom climate scenarios exaggerate trends of the agents that drive global warming. Study of these forcing agents shows that global warming can be slowed, and stopped, with practical actions that yield a cleaner, healthier atmosphere.

Joule

Near-universal recognition of the threat of human-caused global warming, tragically, has not been... more Near-universal recognition of the threat of human-caused global warming, tragically, has not been accompanied by comprehensive quantitative assessment of technological options for mitigating climate change. In this issue of Joule, David W. Keith and co-authors describe a process of capturing CO 2 from the air and provide a much-needed empirical cost estimate based on results from a pilot plant. Estimated costs, exceeding $100 per ton of CO 2 without including the cost of CO 2 storage, are lower than some prior estimates, yet are so high as to strongly support the need for rapid reduction of fossil fuel emissions.

Atmospheric Chemistry and Physics Discussions, 2015

There is evidence of ice melt, sea level rise to +5–9 m, and extreme storms in the prior intergla... more There is evidence of ice melt, sea level rise to +5–9 m, and extreme storms in the prior interglacial period that was less than 1 °C warmer than today. Human-made climate forcing is stronger and more rapid than paleo forcings, but much can be learned by combining insights from paleoclimate, climate modeling, and on-going observations. We argue that ice sheets in contact with the ocean are vulnerable to non-linear disintegration in response to ocean warming, and we posit that ice sheet mass loss can be approximated by a doubling time up to sea level rise of at least several meters. Doubling times of 10, 20 or 40 years yield sea level rise of several meters in 50, 100 or 200 years. Paleoclimate data reveal that subsurface ocean warming causes ice shelf melt and ice sheet discharge. Our climate model exposes amplifying feedbacks in the Southern Ocean that slow Antarctic bottom water formation and increase ocean temperature near ice shelf grounding lines, while cooling the surface ocean...

Reviews of Geophysics, 2010

We update the Goddard Institute for Space Studies (GISS) analysis of global surface temperature c... more We update the Goddard Institute for Space Studies (GISS) analysis of global surface temperature change, compare alternative analyses, and address questions about perception and reality of global warming. Satellite-observed night lights are used to identify measurement stations located in extreme darkness and adjust temperature trends of urban and periurban stations for nonclimatic factors, verifying that urban effects on analyzed global change are small. Because the GISS analysis combines available sea surface temperature records with meteorological station measurements, we test alternative choices for the ocean data, showing that global temperature change is sensitive to estimated temperature change in polar regions where observations are limited. We use simple 12 month (and n × 12) running means to improve the information content in our temperature graphs. Contrary to a popular misconception, the rate of warming has not declined. Global temperature is rising as fast in the past decade as in the prior 2 decades, despite year-to-year fluctuations associated with the El Niño-La Niña cycle of tropical ocean temperature. Record high global 12 month running mean temperature for the period with instrumental data was reached in 2010.