Saturday, February 21, 2015

How pressure-dependent atmospheric warming explains the entire 33C greenhouse effect

Nice to see that others are beginning to appreciate the Maxwell/Carnot/Clausius atmospheric mass/gravity/pressure theory of the greenhouse effect, which completely explains the atmospheric temperature profiles from the Earth's surface all the way to the top of the atmosphere at ~100,000 meters, entirely without radiative forcing from greenhouse gas 'back-radiation'.

The latest is a forthcoming series of articles at the Swedish climate skeptic site Stockholm's Initiative, the first chapter of which is below [Google translation + editing]. These concepts have been discussed in further detail in the series of Hockey Schtick posts on the 'greenhouse equation' and in relation to the Maxwell/Carnot/Clausius atmospheric mass/gravity/pressure theory of the 33K greenhouse effect.

The atmosphere from inside out

02/18/2015 by Goran Ahlgren .

Chapter 1.

So here it is: The sun exposes the soil to shortwave radiation. The annual average value at the "top of the atmosphere" (TOA) is 1366 W / m2 ("the sola constant") calculated on the projected ground disc. The average value over the soil surface, and rotation period 24 hours is 1366/4 = 342 kW / m2.

From the incoming radiation, 30% is reflected (albedo = 0.3) (6% of the atmosphere, 20% of clouds, 4% of the surface). The remainder, 240 W / m2, is absorbed by the atmosphere and the surface. Under the condition of energy balance, the Earth must in turn radiate as much energy.

The sun's electromagnetic spectrum is broadly consistent with the radiation of a black body at 5780 K.

To balance earth's outgoing radiation, in a first approximation, consider the earth as an ideal black body, at a temperature of 255 K. This is usually called the Earth's effective temperature.

An ideal black body requires perfect heat conduction (uniform temperature) within the body. The Earth is not black. It's not a perfect conductor. The temperature is not uniform. The earth is more like a gray insulator. Correction for this provides an effective radiation temperature is a few degrees cooler, 251.95 K. Let us afford ourselves approximation of Earth's effective temperature:

T eff = 252 K

Mean surface temperature is 288 K and 218 K at the so-called tropopause (11,000 m). Troposphere temperature mean value in the vertical direction (Z) is thus

T h = 252 K

This temperature is found at 5500 meters above sea level where the pressure is 0.5 atm. This confirms that the troposphere profiles (L, lapse rate) with respect to temperature and pressure is linear, which we already know from other observations. The function T (Z) [temperature as a function of height Z] can be written

T (Z) = L * Z

where L ELR = 6.5 K / km (Environmental lapse rate), the atmosphere observed temperature profile, is an observable characteristic of the troposphere. It is a consequence of atmospheric thermodynamic properties.

Applying a thermodynamic approach and associated conditions in a dry or humidity wise unsaturated ideal gas under adiabatic conditions (no net flow of heat to or from the atmosphere in a steady state), one can expect

L DALR = g / c p

where L DALR = dry adiabatic lapse rate (DALR), g is the gravitational constant and c p the specific heat. Inserting the current values gives L DALR = 9.8 K / km.

For a moisture saturated gas held at the dew point at increasing height (decreasing pressure), there is power from the water's heat of vaporization. In this case (L MALR , moist adiabatic lapse rate) is not L MALR constant over the height of the atmosphere, but is on average L ALR ≈5 K / km. Reduction of temperature with height in the atmosphere in this case is less because heat of vaporization continuously released by condensation.

In general we can say that the atmosphere observed temperature profile is a consequence of the fundamental thermodynamic conditions, ie atmospheric internal energy constants when the potential energy increases in the gravitational field.

Observed value at lapse rate L ELR = 6.5 K / m2 located in the interval between the two idealized extremes, L March and L DALR . For a hypothetical dry soil with the same effective temperature Teff = 252 K, which is our real earth, one can account with the above reasoning to expect a temperature at sea level at 306 K, 18 K higher than the observed 288 C (252 F + 9.8 K / km x 5.5 km = 306 K). The dry earth L DARL offset and balanced by a heat transfer (cooling) corresponding to 9.8 to 6.5 = 3.3 K / km in the real atmosphere.

The earth's surface and the atmosphere is heated by solar radiation and thus arises a steady state (constant heat content, constant energy flow), sustained by the radiation balance. Included in this condition is of course all internal heat transfer (absorption, emission, conduction, convection, phase transition), which can balance the L adrl that sets the upper limit of the temperature profile. The surface temperature T 0 = 288 K is 35 K above T eff and 17 K lower than the L ADLR , reflecting the balance between the pressure-dependent warming and the cooling chain of heat transfer by conduction, evaporation, convection and condensation, complete with radiation to space.

So far so good! I am convinced that it is possible to find numerous errors, logical gaps and inconsistencies in the above. I am grateful for all the help locating such holes to see if it is possible to fill them. But so far no trace of any greenhouse or re-radiation. We'll see how it goes in the next two chapters will deal with the albedo, the hydrological cycle, the tropopause, discussion of the possible structure of the atmosphere thermostat (the balance between different paths for heat transfer from a control engineering perspective), entropy and other properties of stationary thermodynamic systems far from equilibrium, and the characteristics of other planets atmospheres. Chapter 2, I have promised the editors to report next Friday 27.2.

Tuesday, February 17, 2015

New paper demonstrates East Antarctica was ~3.5-4°C warmer than the present during the last interglacial

A new paper in Climate of the Past Discussions finds from high-resolution ice core data from East Antarctica that temperatures were 3.5-4°C warmer during the last interglacial (~130,000 years ago) than during the present interglacial (the past ~18,000 years).

The IPCC claims warming of over 2°C (an arbitrary figure 'plucked out of thin air') will lead to irreversible and catastrophic "tipping points" or positive feedbacks from which Gaia cannot recover. However, the ice core data from prior interglacials demonstrates this is not the case, and that both Greenland and Antarctica recovered from warming of 8°C and 4°C higher (respectively) during the last interglacial and relative to 1950, and both ice sheets are much larger today than during the past interglacial.

In addition, the ice core data demonstrates that such dramatic changes at the poles occur by entirely natural means and thus, there is no evidence that the (less) dramatic climate changes seen during the current interglacial are unnatural, unusual, or unprecedented.

Second graph from top shows reconstructed temperatures at 2 ice core sites in East Antarctica. Horizontal axis is thousands of years before 1950. Note surface temperatures have warmed about 0.5C since 1950.

Climate dependent contrast in surface mass balance in East Antarctica over the past 216 kyr

F. Parrenin^1,2, S. Fujita^3,4, A. Abe-Ouchi^5,6, K. Kawamura^3,4, V. Masson-Delmotte⁷, H. Motoyama^3,4, F. Saito⁵, M. Severi⁸, B. Stenni⁹, R. Uemura¹⁰, and E. Wolff¹¹
¹CNRS, LGGE, 38041 Grenoble, France
²Univ. Grenoble Alpes, LGGE, 38041 Grenoble, France
³National Institute of Polar Research, Research Organization of Information and Systems, Tokyo, Japan
⁴Department of Polar Science, The Graduate University for Advanced Studies (SOKENDAI), Tokyo, Japan
⁵Japan Agency for Marine–Earth Science and Technology, Yokohama, Japan
⁶Atmosphere and Ocean Research Institute (AORI), University of Tokyo, Chiba, Japan
⁷Laboratoire des Sciences du Climat et de l'Environnement, Institut Pierre Simon Laplace, UMR CEA-CNRS-UVSQ 8212, Gif-sur-Yvette, France
⁸Department of Chemistry, University of Florence, Florence, Italy
⁹Department of Geosciences, University of Trieste, Trieste, Italy
¹⁰Department Chemistry, Biology and Marine Science, Faculty of Science, University of the Ryukyus, Okinawa, Japan
¹¹Department of Earth Sciences, University of Cambridge, UK
Abstract. Documenting past changes in the East Antarctic surface mass balance is important to improve ice core chronologies and to constrain the ice sheet contribution to global mean sea level. Here we reconstruct the past changes in the ratio of surface mass balance (SMB ratio) between the EPICA Dome C (EDC) and Dome Fuji (DF) East Antarctica ice core sites, based on a precise volcanic synchronisation of the two ice cores and on corrections for the vertical thinning of layers. During the past 216 000 years, this SMB ratio, denoted SMB_EDC/SMB_DF, varied between 0.7 and 1.1, decreasing during cold periods and increasing during warm periods. While past climatic changes have been depicted as homogeneous along the East Antarctic Plateau, our results reveal larger amplitudes of changes in SMB at EDC compared to DF, consistent with previous results showing larger amplitudes of changes in water stable isotopes and estimated surface temperature at EDC compared to DF. Within interglacial periods and during the last glacial inception (Marine Isotope Stages, MIS-5c and MIS-5d), the SMB ratio deviates by up to 30% from what is expected based on differences in water stable isotope records. Moreover, the SMB ratio is constant throughout the late parts of the current and last interglacial periods, despite contrasting isotopic trends. These SMB ratio changes not closely related to isotopic changes are one of the possible causes of the observed gaps between the ice core chronologies at DF and EDC. Such changes in SMB ratio may have been caused by (i) climatic processes related to changes in air mass trajectories and local climate, (ii) glaciological processes associated with relative elevation changes, or (iii) a combination of climatic and glaciological processes, such as the interaction between changes in accumulation and in the position of the domes. Our inferred SMB ratio history has important implications for ice sheet mo

Thursday, February 12, 2015

New paper claims AGW pushed the "Western US toward the driest period in 1,000 years"

A modeling study published in Science Advances claims global warming has pushed the "Western US toward the driest period in 1,000 years" and "the U.S. Southwest and Great Plains will face persistent drought worse than anything seen in times ancient or modern, with the drying conditions "driven primarily" by human-induced global warming."

However, the tree-ring proxy data in the paper shows that at the end of the record in ~2002, soil moisture of the central plains was considerably above the average of the past millennium, and peaked around ~1930, a relatively warm period in the US. The proxy record also shows many periods of drought during the Little Ice Age and that the 20th century was relatively wet period in comparison to the past millennium.

For the US Southwest, the proxy data also shows a soil moisture peak around ~1930. If warming is a cause of decreased soil moisture as the paper claims, the proxy data would be expected to show the opposite pattern to that observed. Although the end of the Southwest proxy record in ~2002, conditions were relatively dry, but not as dry as at least 3 other periods during the Little Ice Age. Many other paleoclimate studies have found both droughts and floods were more common during the Little Ice Age in comparison to the 20th century.

Thus, the claim that AGW has "pushed the Western US toward the driest period in 1,000 years" is not supported by the proxy data shown in the paper. In addition, the modeling claim that AGW will cause "unprecedented risk of drought in the 21st century" is entirely based upon overheated climate models which have been falsified at confidence levels exceeding 98%. As shown below, the models did not reproduce the peaks in soil moisture around ~1930 or the peak around ~2000 in the central plains, further evidence that the modeling assumptions are incorrect and the claim of unprecedented drought not supported by observations.

1000 year drought history based on tree rings shown in brown (higher values represent higher soil moisture). Green, red, blue lines are projections from (falsified) climate models.

Warming pushes Western US toward driest period in 1,000 years: Unprecedented Risk of Drought in 21st Century

Date: February 12, 2015

Summary: During the second half of the 21st century, the U.S. Southwest and Great Plains will face persistent drought worse than anything seen in times ancient or modern, with the drying conditions "driven primarily" by human-induced global warming, a new study predicts.

The research says the drying would surpass in severity any of the decades-long "megadroughts" that occurred much earlier during the past 1,000 years -- one of which has been tied by some researchers to the decline of the Anasazi or Ancient Pueblo Peoples in the Colorado Plateau in the late 13th century. Many studies have already predicted that the Southwest could dry due to global warming, but this is the first to say that such drying could exceed the worst conditions of the distant past. The impacts today would be devastating, given the region's much larger population and use of resources.

"We are the first to do this kind of quantitative comparison between the projections and the distant past, and the story is a bit bleak," said Jason E. Smerdon, a co-author and climate scientist at the Lamont-Doherty Earth Observatory, part of the Earth Institute at Columbia University. "Even when selecting for the worst megadrought-dominated period, the 21st century projections make the megadroughts seem like quaint walks through the Garden of Eden."

"The surprising thing to us was really how consistent the response was over these regions, nearly regardless of what model we used or what soil moisture metric we looked at," said lead author Benjamin I. Cook of the NASA Goddard Institute for Space Studies and the Lamont-Doherty Earth Observatory. "It all showed this really, really significant drying."

The new study, "Unprecedented 21st-Century Drought Risk in the American Southwest and Central Plains," will be featured in the inaugural edition of the new online journal Science Advances, produced by the American Association for the Advancement of Science, which also publishes the leading journal Science.

Today, 11 of the past 14 years have been drought years in much of the American West, including California, Nevada, New Mexico and Arizona and across the Southern Plains to Texas and Oklahoma, according to the U.S. Drought Monitor, a collaboration of U.S. government agencies.

The current drought directly affects more than64 million people in the Southwest and Southern Plains, according to NASA, and many more are indirectly affected because of the impacts on agricultural regions.

Shrinking water supplies have forced western states to impose water use restrictions; aquifers are being drawn down to unsustainable levels, and major surface reservoirs such as Lake Mead and Lake Powell are at historically low levels. This winter's snowpack in the Sierras, a major water source for Los Angeles and other cities, is less than a quarter of what authorities call a "normal" level, according to a February report from the Los Angeles Department of Water and Power. California water officials last year cut off the flow of water from the northern part of the state to the south, forcing farmers in the Central Valley to leave hundreds of thousands of acres unplanted.

"Changes in precipitation, temperature and drought, and the consequences it has for our society -- which is critically dependent on our freshwater resources for food, electricity and industry -- are likely to be the most immediate climate impacts we experience as a result of greenhouse gas emissions," said Kevin Anchukaitis, a climate researcher at the Woods Hole Oceanographic Institution. Anchukaitis said the findings "require us to think rather immediately about how we could and would adapt."

Much of our knowledge about past droughts comes from extensive study of tree rings conducted by Lamont-Doherty scientist Edward Cook (Benjamin's father) and others, who in 2009 created the North American Drought Atlas. The atlas recreates the history of drought over the previous 2,005 years, based on hundreds of tree-ring chronologies, gleaned in turn from tens of thousands of tree samples across the United States, Mexico and parts of Canada.

For the current study, researchers used data from the atlas to represent past climate, and applied three different measures for drought -- two soil moisture measurements at varying depths, and a version of the Palmer Drought Severity Index, which gauges precipitation and evaporation and transpiration -- the net input of water into the land. While some have questioned how accurately the Palmer drought index truly reflects soil moisture, the researchers found it matched well with other measures, and that it "provides a bridge between the [climate] models and drought in observations," Cook said.

The researchers applied 17 different climate models to analyze the future impact of rising average temperatures on the regions. And, they compared two different global warming scenarios -- one with "business as usual," projecting a continued rise in emissions of the greenhouse gases that contribute to global warming; and a second scenario in which emissions are moderated.

By most of those measures, they came to the same conclusions.

"The results … are extremely unfavorable for the continuation of agricultural and water resource management as they are currently practiced in the Great Plains and southwestern United States," said David Stahle, professor in the Department of Geosciences at the University of Arkansas and director of the Tree-Ring Laboratory there. Stahle was not involved in the study, though he worked on the North American Drought Atlas.

Smerdon said he and his colleagues are confident in their results. The effects of CO2on higher average temperature and the subsequent connection to drying in the Southwest and Great Plains emerge as a "strong signal" across the majority of the models, regardless of the drought metrics that are used, he said. And, he added, they are consistent with many previous studies.

Anchukaitis said the paper "provides an elegant and convincing connection" between reconstructions of past climate and the models pointing to the risk of future drought.

Toby R. Ault of Cornell University is a co-author of the study. Funding was provided by the NASA Modeling, Analysis and Prediction Program, NASA Strategic Science, and the U.S. National Science Foundation.

Story Source:

The above story is based on materials provided by The Earth Institute at Columbia University. Note: Materials may be edited for content and length.

Journal Reference:
Benjamin I. Cook, Toby R. Ault, Jason E. Smerdon. Unprecedented 21st century drought risk in the American Southwest and Central Plains. Science Advances, 12 February 2015 DOI: 10.1126/sciadv.1400082

Monday, February 2, 2015

New paper finds oceans warming only a tiny 0.002°C-0.005°C/year since 2006

A paper published today in Nature Climate Change claims "Unabated planetary warming...since 2006" of the world's oceans of a tiny 0.005C/year from 0-500 meter depths and an even smaller 0.002C/year for the 500-2000 meter depths. This rate is equivalent to only 0.2°C to 0.5°C ocean warming per century, far less than the 3°C global warming by 2100 central estimate of the IPCC.

Examination of the paper, however, reveals multiple questionable claims and contradictions to the claims of climate alarmists and IPCC:

According to the authors, "the ocean heat gain over the 0-2000 meter layer continued at a rate of 0.4-0.6 W/m2 during 2006-2013." However, according to the IPCC, net anthropogenic forcing is warming the planet at a rate of 1.6 W/m2 or ~3.2 times more than the central estimate of this new paper. This implies a climate sensitivity about 70% less than claimed by the IPCC.
Alarmists claim "90% of the 'missing heat' from greenhouse gases is going into the ocean," therefore, using the central estimate of this paper of a warming rate of 0.5 W/m2, total net anthropogenic forcing of oceans + atmosphere would be 0.5*1.1 = 0.55 W/m2, again far less (66% less) than the 1.6 W/m2 net anthropogenic forcing at present claimed by the IPCC.
The above estimates falsely assume, for the purposes of argument only, that all of the ocean warming is due to increased greenhouse gases. However, IR radiation from greenhouse gases cannot significantly warm the oceans for at least 3 thermodynamic reasons as outlined here and here. Changes in solar insolation modulated by cloud cover and ocean oscillations are not even considered or discussed by this paper as potential mechanisms of the ocean warming patterns noted, but are far more likely to be the cause of any warming observed.
Heat rises, and surface data indicate no global warming for 18+ years. How can zero degrees atmospheric warming cause the oceans up to 2000 meters depth to warm 0.002C/yr? It cannot, without violating thermodynamics.
The uncertainties of measurement of individual ARGO floats are far greater than the claimed warming
Table 1 below shows all of the warming occurred in the Southern Hemisphere 0-60S, whereas the Northern Hemisphere 0-60N actually cooled from 2006-2013. This warming pattern is incompatible with anthropogenic forcing from well-mixed greenhouse gases, which is alleged to be relatively uniform across the planet, and thus the spatially limited warming to the Southern Hemisphere alone is far more likely due to changes in ocean oscillations and/or solar insolation from cloud cover changes.

For these reasons and others, the claim of unabated anthropogenic warming of the oceans from greenhouse gases since 2006 is unwarranted.

Fig TS 5 from the latest IPCC Report claims a continuous net anthropogenic forcing of 1.6 W/m2 at present, far greater than that found by this new paper.

Excerpts:

Sunday, February 1, 2015

New paper finds 'catastrophic collapse of polar ice sheets & substantial sea level rise' up to 11 meters higher than present during the last interglacial

A new paper published in Nature Communications finds the last interglacial period ~127-117 thousand years ago was characterized by "catastrophic collapse of polar ice sheets and substantial sea level rise" of up to 11 meters higher than the present. According to the authors, these climate changes are explained by changes in solar insolation "close to today's value."

Therefore, there is no evidence that the (significantly lower) sea levels and (larger) polar ice sheets of today as compared to the last interglacial are due to man's activity rather than the natural changes expected due to solar insolation changes similar today to the last interglacial.

Further, the authors find no significant changes in seasonality (temperature changes between summer and winter) of the last interglacial compared to the modern seasonality, and attribute such changes to solar insolation similar between the present and last interglacial ~118,000 years ago.

Tropical Atlantic temperature seasonality at the end of the last interglacial

Nature Communications

Article number:

6159

doi:10.1038/ncomms7159

Received: 07 July 2014
Accepted: 22 December 2014
Published: 22 January 2015

Abstract: The end of the last interglacial period, ~118 kyr ago, was characterized by substantial ocean circulation and climate perturbations resulting from instabilities of polar ice sheets. These perturbations are crucial for a better understanding of future climate change. The seasonal temperature changes of the tropical ocean, however, which play an important role in seasonal climate extremes such as hurricanes, floods and droughts at the present day, are not well known for this period that led into the last glacial. Here we present a monthly resolved snapshot of reconstructed sea surface temperature in the tropical North Atlantic Ocean for 117.7±0.8 kyr ago, using coral Sr/Ca and δ¹⁸O records. We find that temperature seasonality was similar to today, which is consistent with the orbital insolation forcing. Our coral and climate model results suggest that temperature seasonality of the tropical surface ocean is controlled mainly by orbital insolation changes during interglacials.

The last interglacial, although not a direct analogue for future climate, has received much attention in the climate-modelling community^1, 2, 3 and has been suggested as a test bed for models developed for future climate prediction^2, 4. This period (~127–117 kyr ago) was characterized by strong orbital insolation forcing⁵, relative warmth⁶ and high sea level⁷. In the Northern Hemisphere, changes in the Earth’s orbit around the sun led to a stronger seasonality of insolation compared to today⁵, which resulted in increased temperature seasonality at the Earth’s surface as inferred from proxy records^8, 9, 10 that commonly represent the time interval of maximum seasonal insolation forcing⁵ between ~127 and ~124 kyr ago. In contrast, the temperature seasonality at the end of the last interglacial (~118 kyr ago), when Northern Hemisphere insolation seasonality was close to today’s value⁵, is not well known. This period that led into the last glacial is particularly interesting as it was characterized by catastrophic collapse of polar ice sheets and substantial sea-level rise^11, 12, abrupt changes in ocean circulation^13, 14and large-scale climate perturbations¹⁵. It has been suggested that the end of the last interglacial may provide clues to a better understanding of the potential for rapid ice-sheet collapse and sea-level rise and, consequently, for abrupt perturbations of the ocean–atmosphere system, under future climate change^11, 12, 14. At the present day, the seasonal temperature changes of the tropical ocean play an important role in seasonal climate extremes such as hurricanes, floods and droughts^{16, 17, 18, 19}. A better understanding of the temperature seasonality ~118 kyr ago is, thus, essential to establish a baseline to evaluate the seasonal response in climate model simulations, for both the end of the last interglacial and for projections of future climate change.

Here we investigate the monthly resolved Sr/Ca and δ¹⁸O environmental proxy signals in a precisely dated shallow-water fossil coral recovered from the southern Caribbean and reconstruct the temperature seasonality in the surface waters of the tropical North Atlantic Ocean at the end of the last interglacial. Sr/Ca variations in aragonitic coral skeletons are a proxy for temperature variability²⁰, which has previously been successfully applied to last interglacial fossil corals^9, 10,21. Coral δ¹⁸O, a proxy that reflects both temperature and seawater δ¹⁸O variations, is used to support our reconstruction. The ²³⁰Th/U method allows precise dating of corals that grew during the last interglacial period²². Our findings indicate that temperature seasonality in the southern Caribbean Sea at 118 kyr ago was similar to today. Our coral records and simulations with a coupled atmosphere–ocean general circulation model indicate an orbital control on temperature seasonality in the tropical North Atlantic at the end of the last interglacial, despite the large-scale perturbations of ocean circulation and climate during this period, and suggest that temperature seasonality of the tropical surface ocean is controlled mainly by orbital insolation changes during interglacials.

Results

Coral preservation and age

The fossil shallow-water coral (Diploria strigosa) was recovered at Bonaire, an open-ocean island in the southern Caribbean Sea, located ~100 km north of South America and ~300 km northwest of the Cariaco Basin (Fig. 1). Bonaire is situated off the South American continental shelf in the northwestward-flowing Caribbean Current, an extension of the Guyana Current that transports equatorial Atlantic surface waters along northeastern South America towards the Caribbean Sea. Thus, sea surface temperature (SST) at Bonaire is representative for a large area of the tropical North Atlantic Ocean²³. Bonaire is influenced by the easterly trade winds, and its present-day climate is semi-arid with an annual precipitation of ~550 mm and the main rainy season during boreal winter²⁴. Bonaire is not influenced by the seasonally migrating Intertropical Convergence Zone (ITCZ) because the northernmost ITCZ position that is reached during boreal summer is located south of Bonaire, over northern South America and the Cariaco Basin²⁵. The fossil coral colony (BON-5-D) was drilled in growth position on top of an elevated reef terrace at the eastern coast of Bonaire (Washikemba). The coral site (68° 11.765′ W, 12° 8.246′ N) is at ~1.5 to ~2.0 m above present sea level, in a distance of ~50 m from the present-day sea cliff. Nearby D. strigosacolonies in growth position (<6 community="" coral="" distance="" fossil="" i="" is="" m="" nbsp="" preserved="" suggest="" that="" this="">in situ

. X-radiography, powder X-ray diffraction, thin-section petrography and scanning electron microscope analysis indicate that the fossil coral is very well preserved (Methods andSupplementary Figs 1–3). ²³⁰Th/U dating yielded a coral age of 117.7±0.8 kyr, showing that the colony grew at the end of the last interglacial period. The initial (²³⁴U/²³⁸U) activity ratio is in agreement with the (²³⁴U/²³⁸U) of modern seawater, providing strong confidence for the reliability of the coral age (Methods and Supplementary Table 1).

Figure 1: Map of the western tropical North Atlantic Ocean.

Coral-based SST seasonality reconstruction

The 118-kyr-old Bonaire coral provides a monthly resolved snapshot of tropical Atlantic SST variability for a time window of 20 years at the end of the last interglacial. This is substantially longer than the only other seasonally resolved snapshot of tropical Atlantic SST for the last interglacial, an ~5-year record of a 127-kyr-old coral from Isla de Mona (67.9° W, 18.1° N) in the northern Caribbean Sea⁹. Our Bonaire monthly resolved coral Sr/Ca- and δ¹⁸O-SST reconstructions show clear annual cycles in both proxies (Fig. 2a,b), giving additional confidence that the analysed coral skeleton was not subject to diagenetic alteration. The Sr/Ca-SST reconstruction indicates a seasonality of 2.6±0.1 °C (±1 s.e.) at 118 kyr ago (Fig. 2a,e). Monthly resolved records of three modern Bonaire D. strigosa corals satisfactorily document the instrumental SST²⁶ seasonality of 2.9±0.1 °C (±1 s.e.; 1910–2000), indicating a reconstructed modern Sr/Ca-SST seasonality that ranges from 2.4±0.3 °C (±1 s.e.) to 3.0±0.3 °C (±1 s.e.) for time intervals of the last century, resulting in a reconstructed modern mean seasonality of 2.8±0.4 °C (±1 s.d.; ref. 23; Fig. 2c,e). Taking into account these differences in the reconstructed SST seasonality among the three modern corals indicates that the reconstructed SST seasonality of 2.6±0.1 °C (±1 s.e.) at 118 kyr ago, at the end of the last interglacial, is not significantly different from today (Methods and Supplementary Note 1).

Figure 2: Tropical North Atlantic coral-based temperature seasonality.

The coral δ¹⁸O-SST reconstruction for 118 kyr ago indicates a seasonality of 2.4±0.1 °C (±1 s.e.), which is very similar to the Sr/Ca-based seasonality estimate of 2.6±0.1 °C (±1 s.e.; Fig. 2a,b,e,f). Thus, the coral δ¹⁸O seasonality at 118 kyr ago may be attributed mainly to the seasonality of SST. This is broadly in line with the modern situation²⁷, where the mean SST seasonality reconstructed by coral δ¹⁸O of 2.3±0.3 °C (±1 s.d.) is slightly reduced (by ~0.5 °C, not correcting for seasonal seawater δ¹⁸O changes) relative to the Sr/Ca- and instrument-based estimates (Fig. 2e,f), most likely owing to hydrologic cycle effects such as the Bonaire winter rainfall regime²⁴. The coral δ¹⁸O-SST reconstruction supports our major finding based on coral Sr/Ca, and both proxies indicate SST seasonality in the southern Caribbean Sea at the end of the last interglacial similar to today. Consequently, both proxies may also suggest a Bonaire hydrologic cycle similar to today at 118 kyr ago. Crucially, our results are robust towards the choice of the coral Sr/Ca-SST and δ¹⁸O-SST relationships, which affect mainly the absolute magnitude of reconstructed SST seasonality but have only minor effect on the relative seasonality estimates among corals, and we would have reached identical conclusions using other relationships (Supplementary Fig. 4).

Discussion

The annual SST cycle in the Caribbean Sea, with a minimum in boreal winter/spring and a maximum in boreal summer/fall, follows primarily the annual cycle of insolation^28, 29. Bonaire monthly coral Sr/Ca records for snapshots since the mid-Holocene, comprising a total length of 295 years, suggest that the SST [sea surface temperature] annual cycle in the southern Caribbean Sea has not substantially changed, with the exception of a time interval at 2.35 kyr ago²³ (Fig. 2e). Disregarding the 2.35 kyr coral, a trend towards lower SST seasonality during the time interval 6.22–1.84 kyr ago may be inferred from the coral Sr/Ca records, as well as a slightly but significantly higher SST seasonality than the present day at 6.22 kyr ago. Such an evolution through time would be consistent with an orbital insolation control on Holocene SST seasonality in the southern Caribbean Sea (Fig. 3a), which is supported by simulations with a coupled atmosphere–ocean general circulation model (Community Earth System Models, COSMOS; Methods and Fig. 3b). However, we note that the magnitude of the trend in the fossil coral data is minor, close to the ±1 s.d. range of the modern mean Sr/Ca-SST seasonality reconstructed from three modern corals (Fig. 2e), which may also reflect the relatively small magnitude of insolation-controlled SST seasonality changes at lower latitudes throughout the Holocene (Fig. 3a,b).

Figure 3: Tropical North Atlantic insolation and temperature changes.

Similarly, the coral δ¹⁸O-SST reconstruction²⁷ reveals a trend towards lower seasonality during the time interval 6.22–1.84 kyr ago, which is more pronounced compared with the trend that may be inferred from coral Sr/Ca, as well as a substantially and significantly higher seasonality than the present day at 6.22 kyr ago (Fig. 2f). This evolution of coral δ¹⁸O seasonality through time is consistent with an insolation control on Holocene SST seasonality in the southern Caribbean Sea (Fig. 3a,b). Differences between the coral δ¹⁸O-SST and Sr/Ca-SST seasonality estimates (Fig. 2e,f) reflect primarily seasonal changes in seawater δ¹⁸O; however, we note that reconstructions of seawater δ¹⁸O seasonality can be sensitive towards the choice of the coral δ¹⁸O-SST and Sr/Ca-SST relationships (Supplementary Fig. 4). However, for 6.22 kyr ago, an anomalous seawater δ¹⁸O seasonality may be inferred from the coral records that could be explained by hydrologic cycle effects such as, among others, Bonaire summer rainfall²⁷, which would be contrary to the present-day winter rainfall regime²⁴ (Supplementary Fig. 4). This interpretation would be in line with reconstructions of increased summer rainfall over northernmost South America during the early to mid-Holocene, owing to a more northerly position of the boreal-summer ITCZ³⁰ and possibly paired with a thermodynamic increase in rainfall because of strengthening local summer insolation³¹. We note that the subsequent southward migration of the boreal-summer ITCZ over the course of the Holocene that was controlled by orbital insolation changes^30, 31 is also in line with the trend towards lower coral δ¹⁸O seasonality over this time interval (Fig. 2f).

The significantly increased SST seasonality at 2.35 kyr ago, indicated by coral Sr/Ca (Fig. 2e), may be related to internal climate variability and is interpreted to reflect a time interval of strengthened El Niño-Southern Oscillation (ENSO) teleconnections to the Caribbean region²³, probably modulated by the North Atlantic Oscillation (NAO). This interpretation is broadly in line with the present-day modulation of southern Caribbean SST seasonality by ENSO teleconnections^23, 32, which vary in strength on interdecadal timescales and are modulated by the NAO³³. Indeed, pronounced interannual variability at a period of 5.7 years in the Sr/Ca record of the 2.35 kyr coral²³, the most prominent period in the cospectrum of the instrumental indices of ENSO and NAO^34, 35, may be indicative of pronounced ENSO–NAO interactions at that time²³. Importantly, the strength of the ENSO phenomenon in the tropical Pacific did not change markedly around 2.3 kyr ago³⁶. We note that the increased coral Sr/Ca-SST seasonality at 2.35 kyr ago is not accompanied by an increased coral δ¹⁸O-SST seasonality (Fig. 2e,f), which suggests an anomalous seawater δ¹⁸O seasonality that could be explained by hydrologic cycle effects such as, among others, increased Bonaire winter rainfall (Supplementary Fig. 4). This interpretation would be broadly in line with the present-day modulation of Bonaire climate by ENSO teleconnections, where La Niña events result in increased SST seasonality through anomalous winter cooling²³ as well as in increased winter rainfall²⁴.

Our coral-based finding of SST seasonality similar to today in the southern Caribbean Sea at 118 kyr ago (Fig. 2e) is consistent with an insolation seasonality at the latitude of Bonaire that was close to today’s value (Fig. 3a). This result could be interpreted in a way that southern Caribbean SST seasonality at that time was controlled mainly by orbital insolation changes. Simulations performed with a coupled atmosphere–ocean general circulation model (COSMOS) support this interpretation (Methods). The modelled changes in southern Caribbean SST seasonality at Bonaire throughout the last interglacial follow largely the variations in insolation forcing over the time interval 130–115 kyr ago (Fig. 3). Moreover, the modelled global surface air temperature anomaly indicates that temperature seasonality in the southern Caribbean at 118 kyr ago is part of a hemisphere-scale pattern that can be attributed largely to insolation forcing (Supplementary Fig. 5). Additional model simulations with freshwater forcing to mimic an abrupt ice-sheet collapse and weakening of the North Atlantic thermohaline circulation at 118 kyr ago or with reduced Greenland ice sheet and dynamic vegetation reveal very similar results (Methods), indicating no significant impact on southern Caribbean SST seasonality (Fig. 3 and Supplementary Figs 5 and6). Thus, our model-based results strongly suggest that SST seasonality in the tropical North Atlantic Ocean at the end of the last interglacial was controlled mainly by orbital insolation changes. Although the slightly lower modelled SST seasonality at 118 kyr ago relative to today (Fig. 3b) appears to be consistent with the coral Sr/Ca-SST seasonality estimate for the end of the last interglacial (Fig. 2e), we consider the latter as similar to today as a result of our uncertainty assignments that take into account the differences in the seasonality estimates among the three modern corals (Methods).

The relatively stable SST seasonality in the tropical North Atlantic Ocean at the end of the last interglacial and its inferred orbital control is remarkable as this period was characterized by large-scale perturbations of ocean circulation and climate resulting from instabilities of polar ice sheets^{11, 12, 13, 14, 15}. Results from Western Australia suggest that, after a prolonged period of stable sea level at ~3–4 m above present sea level between 127 and 119 kyr ago, eustatic sea level rose rapidly to ~8 m above present at the end of the last interglacial, peaking at 118.1±1.4 kyr ago¹², which is contemporaneous with the age of our southern Caribbean coral (117.7±0.8 kyr; Fig. 4b). It has been suggested that this substantial jump in sea level at the end of the last interglacial resulted from collapse of the Greenland and particularly Antarctic ice sheets, after a critical ice-sheet stability threshold was crossed¹². Such an event may have had substantial impacts on global ocean circulation and climate. Interestingly, varved lake sediments in central Europe indicate an extreme 468-year arid and cold event at 118 kyr ago (Fig. 4d), which has been interpreted to result from a sudden southward shift of the warm North Atlantic drift¹⁵. Furthermore, western North Atlantic sediments indicate an abrupt ~400-year deep-water reorganization event at ~118 kyr ago associated with changes in the thermohaline circulation¹³ (Fig. 4e), which has been interpreted to mark the beginning of climate deterioration at the end of the last interglacial¹³. Recent evidence suggests even two events of substantial North Atlantic deep-water reduction at the end of the last interglacial, at ~119.5 and ~116.8 kyr ago¹⁴ (Fig. 4c).

Figure 4: Bonaire coral age and last interglacial sea level and climate change.

Our findings based on combining coral proxy records with climate model simulations indicate that northern tropical Atlantic SST seasonality at 118 kyr ago was similar to today and controlled mainly by orbital insolation changes, despite dramatic ocean circulation and climate perturbations resulting from instabilities of polar ice sheets that characterized the end of the last interglacial^{11,12, 13, 14, 15}. Today, tropical Atlantic SST plays a major role in seasonal climate extremes, such as hurricanes, flashfloods and droughts^{16, 17, 18, 19}, which cause severe socioeconomic damage on the adjacent continents. Our results indicate that SST seasonality in the tropical Atlantic did not substantially change during a period of abrupt high-latitude ice sheet, ocean and climate perturbations at the end of the last interglacial, and, thus, suggest that tropical SST seasonality is controlled mainly by orbital insolation changes during interglacials. However, more seasonally resolved proxy records of SST are needed to better constrain both the climate sensitivity of the tropical ocean in the past and the seasonal response in model-based scenarios of past and future climate change.