CGD 2008 Profiles in Science: Climate Change Research

The Climate Change Research Section (CCR) is part of the Climate and Global Dynamics (CGD) Division at the National Center for Atmospheric Research (NCAR). The CCR section makes extensive use of state-of-the-art coupled climate system models to study the sensitivity and stability of the Earth system to a variety of forcings, including changes of greenhouse gases, aerosols, solar irradiance, volcanic forcing, land characteristics, and land use change. CCR is a focal point for NCAR and university paleoclimate research and serves as a resource to the paleoclimate and climate change research community in the use of the Community Climate System Model (CCSM). CCR scientists collaborate closely with major U.S. Department of Energy (DOE) laboratories in developing and using high-performance coupled climate models to address national and international climate research and climate change policy questions.

Paleoclimate

Paleoclimate research is an essential part of studying Earth's changing climate. The historic and geologic record of Earth’s past provides a unique source of information for climate change research. These records tell us that Earth's climate experienced changes on many time scales, and that these changes involved the interaction of many Earth system processes. Our ability to accurately simulate Earth’s past is a necessary test for climate models. Given the vast span of time in Earth's history, NCAR's paleoclimate research is divided into three time regimes: 1) the past centuries to millennia, 2) the period spanning from the past few million years to the past few thousand years, 3) the period spanning over millions of years of Earth history. NCAR paleoclimate research focuses on using the Community Climate System Model to simulate climates of these three time regimes. Scientists at NCAR work closely with university colleagues to compare model simulations with observational records. The synergy between simulations and observations provides a unique framework for exploring Earth’s climate system. This research is highly relevant for studying Earth’s future climate, given that the projected greenhouse forcing over the next century is similar to that which existed in Earth’s past.

High-Resolution Climate of Past Centuries and Millennia

Global scale climate change has been detected and attributed to anthropogenic forcing. But how well are current climate models ready to project or predict regional climate over the coming decades and beyond? Two fundamental issues are limiting this necessary next step. First, regional climate generally exhibits much more variability and records are often too short to confidently identify the source of the variations. Second, emphasis in climate model development has mostly been put on reproducing the mean annual cycle, but the next challenge is to faithfully reproduce the range of variability.

Caspar Ammann's research focuses on the necessary next steps of extending the instrumental records through the development of improved high resolution climate reconstructions and of strategically designing climate model studies to understand the processes that are dominating the sub-continental scale of climate variability.

Using a new collection of high resolution sulfate records in polar ice cores, novel volcanic forcing series are being developed that are more accurate in estimating the magnitude of past forcing, but for the first time also quantify the uncertainty of eruption occurrence and magnitude. The distribution of frequency and magnitudes are then simulated and possible future volcanic forcing scenarios can be derived, an aspect of future climate variability largely ignored thus far.

A fundamental problem in identifying what part of climate variability is externally forced arises from the large uncertainty in existing reconstruction methods and series. Amman's collaborations with a multi-institution team of paleoclimatologists and statisticians is developing a new way of reconstructing climate using Bayesian Hierarchical Models as the framework. This allows for a more complete exploitation of the available climate record through inclusion of records with extremely different characteristics as well as explicit physical constraints. Additionally, the community program of the Paleoclimate Reconstruction (PR) Challenge is setup to test the accuracy of regional reconstructions and to guide the paleo reconstruction communities in developing more adequate forward models for their proxies. Using climate model output, a systematic intercomparison of the existing reconstruction approaches is performed and a double-blind setup will allow the community to identify where the next efforts need to be put in.

Directly embedded in these regional climate analyses are comparisons between real world and model-based regional climate signals that serve as a fundamental test of climate models ability to reproduce the forced aspects of regional climate. Collaborations across ESSL are used to identify what model configurations are necessary to capture the necessary processes. Using a suite of simulations with increasing model complexity from the standard CCSM towards a more complete representation of the climate system of WACCM (vertical extent, coupled chemistry), climate response to the repeated solar cycle and to the injection of volcanic aerosol are investigated.

Earth’s Climate from the Pliocene to Holocene

There are still many gaps in our understanding of future climate change, including the stability of the polar ice sheets and consequences for sea level, the regional responses of the hydrologic cycle and vegetation, and feedbacks of carbon dioxide and methane with the ocean and land as climate changes. We can inform the future on these questions by understanding the past. NCAR paleoclimate modeling in close coordination with proxy data reconstructions integrates Earth as a system, currently looking at the interactions among the atmosphere, ocean and land surface, but progressing now to include interactions with biology, chemistry, and ice sheets.

Glacial-Interglacial Simulations

Building on successful "snapshot" simulations with CCSM, glacial-interglacial simulations being done by Bette Otto-Bliesner and Esther Brady are now focusing on transient climate changes. Our CCSM3 simulation for the Last Glacial Maximum (LGM) included many successes, including a realistic reproduction of the deep ocean temperature and salinity structure and thermohaline circulation, with sea ice controls on water mass formation in both hemispheres important; and tropical cooling matching that recorded by planktonic foraminifera in the Indian and Western Pacific Oceans indicating that CCSM3 has about right the tropical climate sensitivity to CO₂. With these successes we received a large computing grant from DOE INCITE to run the first set of synchronously coupled transient ocean-atmosphere-dynamic vegetation GCM simulations of the past 21,000 years (TraCE-21) using CCSM3. The TraCE-21 simulation, in collaboration with Zhengyu Liu and Anders Carlson (U. Wisconsin), Peter Clark (Oregon State), and Rob Jacobs and Dave Erickson (DOE), marks a new era in paleoclimate model-data comparison by allowing for a direct comparison of time series between model and data. It also provides a strong test on CCSM for its climate sensitivity to various forcings, especially, the greenhouse forcing, as well as its capability for the simulation of abrupt climate changes. Otto-Bliesner and Nan Rosenbloom, collaborating with Carrie Morrill and Amy Wagner (CIRES), are also looking in detail at the 8.2ka event, an abrupt climate change associated with anomalous freshwater flow into the North Atlantic.

Workshops are being developed together with our university colleagues to connect scientists, worldwide and across disciplines, to allow intercomparisons and synthesis of these simulations with proxy data. A series of two workshops, SynTraCE-21000, have been planned to prepare a three-dimensional synthesis and database of the transient evolution of the Earth system over the last 21,000 years. The first workshop with over 20 university researchers was held 10-13 August in Madison; the second workshop will be held in summer 2009 at NCAR. We are also active participants in PMIP with our simulations for LGM and mid-Holocene available on the PMIP database for access worldwide. Bette Otto-Bliesner is also organizing the next PMIP2 Workshop, to be held 13-19 September in Estes Park, where over 70 international participants will discuss future model-data intercomparison projects and develop a White Paper of proposed paleoclimate simulations for AR5.

Mid-Pliocene Simulations

In 1999 the United States Geological Survey (USGS) created PRISM2 (Pliocene Research, Interpretation and Synoptic Mapping datasets, version 2), a set of gridded datasets for sea surface temperatures, sea ice, geography, and vegetation for the mid-Pliocene (~3 million years ago). This period is possibly the closest paleo analog for the equilibrium climate with current CO₂ levels. A CAM/CLM simulation has been run for this time period by Bette Otto-Bliesner and Nan Rosenbloom and intercompared to similar simulations with the GISS and HadAM models. In addition, working with SOARS student Zi Zi Searles of San Francisco State University, sensitivity simulations have been run warming sea surface temperatures even more, based on new proxy observations, for the California, Peruvian, North African, and South African margin. These simulations show the importance of warm sea surface temperatures off the coast of California for correctly simulating proxy indicators of a wetter western US during the mid-Pliocene. These simulations fit closely with future themes of the Paleoclimate Modeling Intercomparison Project (PMIP).

PMIP and IPCC

The Paleoclimate Modeling Intercomparison Project (PMIP) is a long standing initiative endorsed by both WCRP and IGBP, coordinating paleoclimate modelling activities that provide valuable information on the mechanisms of climate change, the identification of key feedbacks operating in the climate system and, through model evaluation, the capability of climate models to reproduce climates different from today. PMIP results have been used extensively in past IPCC assessments. At its next workshop in September 2008, paleo scientists will identify key climate targets for model simulations and data synthesis that can help reduce uncertainties in future climate projections. Specific simulations of key past time periods will be proposed to the WCRP WGCM as potential contributions of PMIP to the IPCC Fifth Assessment (AR5).

Earth’s Climate in Deep Time

Geological and geochemical data indicate that Earth experienced climates that were significantly warmer and colder compared to the present. Simulating Earth’s climate for different geologic times allows one to study both forcings and feedbacks that establish and maintain these climate regimes. Studying climates for different time periods also allows one to look at the sensitivity of the system on a wide range of time scales. Finally, our ability to accurately simulate deep time climates under extreme forcing conditions increases our confidence in applying climate models to simulating future climate.

Deep Time Modeling Activities

NCAR's Deep Time Paleoclimate research activities are a part of the Community Climate System Modeling (CCSM) activity. A paleoclimate version of the CCSM is maintained within the Climate Change Research Section of CGD that can be applied to a wide range of deep time climate problems. Application of the CCSM to deep time climates is an important component of the overall CCSM activity. If the CCSM can accurately simulate past climate for a wide range of forcings, then one has additional confidence that the model can be used to look at future climate states. Jeffrey Kiehl and Christine Shields lead the deep time modeling activities. Christine Shields is the deep time liaison to the larger community. Simulating deep time climates extends beyond modeling the physical climate system. Kiehl and Shields have collaborated with Jean-Francois Lamarque (ACD) to model the three dimensional atmospheric chemical state of the Latest Permian (251 Ma) and the Paleocene Eocene Thermal Maximum (55 Ma) time periods. Collaborations have also been established with Natalie Mahowald (formerly of CGD, now at Cornell) to model atmospheric dust transport during the mid to late Permian. Kiehl is collaborating with Arne Winguth (U. Texas/Arlington & CGD summer visitor) on the development of an off-line ocean biogeochemical transport model, which can be driven by output from the CCSM and run for million year time scales. The goal of all of these intra-divisional and inter-divisional activities is to develop a hierarchy of modeling tools for deep time research that can be disseminated to the greater scientific community.

Important scientific findings from the current deep time CCSM3 climate simulations include:

Latest Permian simulations that support the hypothesis that enhanced CO₂ greenhouse warming led to global ocean anoxia
Latest Permian atmospheric chemistry simulations indicating that release of H₂S leading to an increased methane lifetime, which would cause a global collapse of the ozone layer
High Resolution (75 km) Latest Permian simulation that indicates the potentially important role of super hurricanes in poleward heat transport
Latest Permian transient simulation indicating the existence of a tipping point in high latitude ocean mixing that occurs around 2 to 3 X PAL CO₂
Paleocene Eocene Thermal Maximum and Latest Permian cloud microphysics simulations indicating the potential role of reduced cloud condensation nuclei in increased polar warming

Deep Time Community Outreach

The purpose of the outreach activity is to establish a closer connection between the geological communities and climate modeling community. There is a great need to facilitate closer interaction between these two communities to evaluate climate model simulations with field data. The global coverage of climate models can also act as a bridge among the various pieces of geological data, which can be invaluable in discovering how past climates originated and were maintained for long time periods. The NSF funded deep time liaison works to bring these communities together. This goal is achieved by supporting the university community in: 1) transferring existing output from CCSM deep time simulations to university scientists, 2) mining the CCSM output for specific site comparisons, 3) helping in configuring the CCSM for university scientists, graduate students and post-docs to run the model at NCAR, and 4) directly collaborating with Kiehl and Shields on CCSM paleoclimate simulations at NCAR. Over the past year and a half, Kiehl and Shields have established a number of connections to the university community.

More recently, NSF has offered to fund a series of deep time climate workshops at NCAR. These workshops will be held annually, and will focus on a particular time period. The purpose of the workshops will be to bring the geological and geochemical communities in direct contact with CCSM climate modeling. An important component of these workshops will be to train graduate students in the use of model simulations for their geological research. Kiehl has agreed to coordinate the workshops with the help of university collaborators. The first workshop will take place in 2009 and will focus on the climate at the Permian-Triassic boundary. Future workshops will be held on climates of the PETM, the Cretaceous, and the Latest Ordovician.

Recent publication information can be found below:

Present and Future Climate Change Research

The CCR Section has a Climate Change and Prediction (CCP) group which focuses on present and future climate change which is primarily funded by the DOE's Office of Biological and Environmental Research program which has a Climate Change subprogram with one of its goals being to predict accurately any global and regional climate change induced by increasing atmospheric concentrations of aerosols and greenhouse gases. This is DOE's contribution to the U.S. Climate Change Science Program that integrates federal research on global change and climate change. CCP has concentrated on the decade to centuries time scales goal which is part of the priorities for the federal Climate Change Science Program (CCSP).

Recent publication information can be found below:

Planning for the IPCC Fifth Assessment Report

We plan to apply the CCSM4 Earth System Model to perform experiments with the new Representative Concentration Pathway (RCP) mitigation/adaptation scenarios for the IPCC Fifth Assessment Report (AR5). This model includes the detailed physical, chemical, and biological processes, interactions and feedbacks in the atmosphere, oceans, and land surface, to enable us to perform experiments with the policy-relevant RCP adaptation/mitigation scenarios. Similar to the Fourth Assessment we plan to provide to the climate change research community one of the largest set of computer generated ensemble simulations over the next few years. In addition to performing the simulations we will take a lead role in the analyses of the climate change simulations. Another aspect of the coordinated activities for the AR5 will be decadal predictability/prediction experiments. We intend to use the 50 km resolution CCSM4 for these experiments to quantify decadal predictability in ^th century hindcasts, and to perform future predictions for the 2030 time frame.

Some Research Highlights

Permafrost Changes

Simulations by global climate models show that when sea ice is in rapid decline, the rate of predicted Arctic warming over land can more than triple. The image at left shows simulated autumn temperature trends during periods of rapid sea-ice loss, which can last for 5 to 10 years. The accelerated warming signal (ranging from red to dark red) reaches nearly 1,000 miles inland. In contrast, the image at right shows the comparatively milder but still substantial warming rates associated with rising amounts of greenhouse gas in the atmosphere and moderate sea-ice retreat that is expected during the 21^st century. Most other parts of the globe shown in white will experience warming, but at a lower rate of less than 1 degree Fahrenheit (0.5 Celsius) per decade. (Image by Steve Deyo, ©UCAR.)

Storm Track Changes Over the U.S.

Haiyan Teng has examined future changes in the wintertime storm activity over North America in CCSM3 (see Teng, Washington, and Meehl, 2008). Over the Pacific Ocean, most other members also produce suppressed baroclinicity in the North Pacific and enhanced baroclinicity in the subtropical eastern Pacific, accompanied with a southward shift and an eastward extension of the Pacific westerly jet. Over the Atlantic, the nine-member mean 300 hPa zonal wind anomalies are characterized by enhanced westerlies across the Caribbean extending to the subtropical Atlantic.

NCAR's Contribution to the Earth System Grid

NCAR's most-advanced global coupled climate model, CCSM, provided the largest contribution from any single climate model to the IPCC Fourth Assessment Report. Approximately 120 TB of original CCSM data was processed to create about 12 TB of IPCC-compliant data, a process that took 15 months. Additionally, CCSM data is available to the wider community and the general public via the Earth System Grid (ESG) project. Over 200 TB of CCSM data can be downloaded from the ESG website at NCAR, and over 1,000 individuals have registered with ESG to access CCSM data. The CCSM source code itself, also on the ESG system, has been accessed more than 700 times. In terms of volume, in July 2008, more than 6 TB (an average of over 200 MB per day) of CCSM output data was retrieved from the NCAR ESG portal. We anticipate for the fifth IPCC assessment that we will provide to the climate change research community many times the data we provided to the fourth assessment.

Low Emission Scenario: How Much Global Warming Can Be Avoided By Mitigation

We have explored the question of how much regional climate change can be avoided by a large reduction in emissions of greenhouse gases and whether the global temperature can be stabilized. This research addresses that question by computer simulating climate change with a new low 21^st century greenhouse gas scenario. The research also examines the climate impacts. When compared to a non-intervention reference scenario, the world requires emission reductions of 67% by 2100 to prevent about half of the changes in temperature and precipitation. This would also stabilize global temperature, Arctic sea ice and permafrost. The intensity of heat waves would be 55% less; however, the sea level would continue to rise from thermal expansion of oceans and glacial melt for many centuries after the 2100.

CGD Research Catalog