ICTS

The earth’s climate has undergone many fluctuations from decadal to million-year timescales. The present concern is regarding the impact of human actions on the earth’s climate. Can human actions lead to tipping the earth into a new warmer climate state? To understand the
various modes of climate variability, it is important to look at the past climate variations from proxy data. The proxy data reveal that earth’s climate was quite stable for about 10,000 years before the advent of the industrial civilization. The earth’s climate showed dramatic fluctuations when it emerged from the last glacial age to the present interglacial on account of rapid changes in oceanic thermohaline circulation. An understanding of the factors that caused these fluctuations is necessary (but not sufficient) to predict how climate will change in the 21 st century in response to human actions. The rate of increase of carbon dioxide and temperature in the latter part of the 20 th century and early 21 st century is more than ten times higher than what has been observed during the past million years. Hence one must be prepared for abrupt changes in the earth’s climate. 

Climate change has emerged as one of the most formidable challenges facing society in the 21st century. The emission of greenhouse gases by human activities is warming the planet. There are two ways to deal with this warming: mitigate it by reducing the emissions or adapt to the impacts of warming. In practice, we need to both, because some of the climate impacts have already occurred and more will inevitably occur even in the best mitigation scenarios. Therefore, we need to predict how different components of the climate system will evolve to efficiently plan for mitigation and adaptation. Earth system models (ESMs) are the tools we use for to make these predictions. In this talk, we begin with a brief historical overview of these models and conclude with a survey of the current state-of-the-art in modeling the earth system. Current and upcoming developments will be discussed, including better representations of ocean and ice processes, cloud-resolving spatial grids, and potential applications of machine learning.

Atmospheric aerosols from human activities and natural sources have significant effects air quality, climate, and public health. Global climate models (GCMs) are an essential tool for studying climate, and almost all GCMs have included the representation of aerosols. However, aerosol radiative forcing from GCMs is one of the large uncertainties in their projection of climate change according to the IPCC reports. I will start my talk by asking the two questions: (1) What are main sources and sinks of the sulfate and black carbon lifecycles, respectively? (2) What roles do these aerosols play in the Earth’s climate system? I will then talk about what are aerosols, their sources, and characteristics of aerosol physical, chemical and optical properties (e.g., size, mixing status, composition, etc.) I will talk about the representations of aerosol in GCMs, including bulk, modal and sectional methods. I will discuss about the life cycles of aerosols in the atmosphere, including emission, gas- and aqueous-phase production, nucleation, coagulation, water uptake, and dry and wet removal from the atmosphere. I will highlight the most uncertain processes treated in GCMs. Finally, the future research needs of improving the aerosol representations in GCMs are pointed out.

In this talk, I will first attempt engage the participants in current approaches used to isolate climate influences of atmospheric aerosols, experienced by society as air pollution. Combined analysis of observational data and global climate model simulations will be presented. Using these approaches, our recent work has uncovered evidence to link the enhancement of atmospheric aerosols to the suppression of monsoon rainfall and intensification of heat waves over India. We also reveal changes in cloud properties, linked to the radiation balance and rainfall. Highlights of underlying mechanisms and attempts to articulate unexpected or less understood behaviour and elements would reveal knowledge gaps. Discussion and participant questions are anticipated to suggest links to theoretical knowledge, which could feed into current approaches to strengthen our understanding of climate change.

Study materials provided:
Muduchuru, K., C. Venkataraman (2021) Influence of aerosols spatial heterogeneity of atmospheric static energy and stratiform rainfall response over India in the ECHAM6-HAM2 GCM, Climate Dynamics, https://doi.org/10.1007/s00382-021-05908-4.

Mondal, A., N. Sah, A. Sharma, C. Venkataraman and N. Patil (2020) Absorbing aerosols and high temperature extremes in India: a general circulation modelling study, Int. J. Climatol., https://doi.org/10.1002/joc.6783.

Dave, P., C. Venkataraman, M. Bhushan (2020) Absorbing aerosol influence on temperature extreme events: An observation based study over India, Atmos. Environ., https://doi.org/10.1016/j.atmosenv.2019.117237.

Patil, N. C. Venkataraman, K. Muduchuru, S. Ghosh, A. Mondal (2018) Disentangling sea- surface temperature and anthropogenic aerosol influences on recent trends in South Asian monsoon rainfall, Climate Dynamics, https://doi.org/10.1007/s00382-018-4251-y.

Dave, P., M. Bhushan and C. Venkataraman (2017) Aerosols cause intraseasonal short-term suppression of Indian monsoon rainfall, Scientific Reports, 7: 17347, DOI:10.1038/s41598-017-17599-1.

Patil, N., P. Dave, C. Venkataraman (2017) Contrasting influences of aerosols on cloud properties during deficient and abundant monsoon years, Scientific Reports, 7, 44996, doi: 10.1038/srep44996.

Traditional general circulation models, or GCMs -- i.e. 3D dynamical models with vertical ""columns"" containing unresolved terms represented in equations with tunable parameters -- have been a mainstay of climate research for several decades, and some of the pioneering studies have recently been recognized by a Nobel prize in Physics. Yet, there is considerable debate around their continuing role in the future. Frequently mentioned as limitations of GCMs are: the non-local nature of certain unresolved processes in atmosphere and ocean, which do not fit readily into a GCM's ""column"" abstraction for unresolved scales; the ""structural"" uncertainty across models with different representations of unresolved scales; and the fact that the models are tuned to reproduce certain aspects of the observed Earth. It is now often contended that nothing short of resolving finer-scale motions, coupled with assimilation of present-day observations to control model biases, will in fact address these shortcomings, and that a future generation of models -- including what are sometimes referred to as ""digital twins"" -- will address these issues through substantially higher resolution. At the same time, models used in service of decision-making and policy, including the recently concluded IPCC AR6, rely on emulators allowing of rapid exploration of multiple future scenarios
through the use of statistical techniques connecting forcings to outcomes, and for the correction of biases. Furthermore, it is noted that models pegged to present-day climate do not do a good job of representing the climate fluctuations of the past, including past warm climates that may hold lessons for the climate emergency.

In this talk, I argue that the sense that GCMs may be obsolescent comes from these conflicting demands: very high resolution for some key processes, which restrict the ability to explore and quantify uncertainties and study low-frequency variability; the need to explore many counterfactual scenarios for constructing climate policy, which cannot be guided simply by present-day observations; the long simulation times needed to understand prior episodes of abrupt climate change and global warming. We present potential approaches founded on a renewed emphasis to mathematical methods of calibration and dimensionality reduction, using the GCM's structure to connect twins to emulators to long-running models of the Earth's past and future.

Monsoons play an important role in the variability of the climate in the tropics. Monsoon rainfall fluctuates on many time scales ranging from weeks, seasonal , interannual, interdecadal to centennial. Will these modes of variability change on account of global warming? The data from rain gauges cannot be used to examine changes in monsoon on a centennial scale. We can use proxy data from ice core, ocean sediments and caves to derive the variation in rainfall during the past 100,000 years. These data reveal how changes in solar radiation, water vapor carbon dioxide and sea ice cover influenced the variability of monsoons on many time scales. The data show that Halley’s land-sea contrast theory of monsoon proposed in 1686 is wrong. A new paradigm based on energy and moisture budget highlights the role of water vapor and the net radiation at the top of the atmosphere in regulating monsoon rainfall. Coupled ocean-atmosphere models are able to simulate these changes and hence can be used to unravel the mechanism governing changes in monsoon on long time scales. This will help us to predict how monsoon rainfall will evolve in the next hundred years.