Scientific understanding of the factors and processes that govern the evolution of Earth's climate has increased markedly over the past several decades, as has the ability to simulate and project future changes in the climate system. As noted in Chapter 2, this knowledge has been regularly assessed, synthesized, and summarized by the Intergovernmental Panel on Climate Change (IPCC), the U.S. Global Climate Research Program (USGCRP, referred to as the U.S. Climate Change Science Program from 2000 to 2008), and other groups to provide a thorough and detailed description of what is known about past, present, and projected future changes in climate and related human and environmental systems. This chapter provides an updated overview of the current state of knowledge about the climate system, followed by a list of some of the key scientific advances needed to further improve our understanding.
To help frame the sections that follow, it is useful to consider some questions that decision makers are asking or will be asking about changes in the climate system:
• How are temperature and other aspects of climate changing?
• How do we know that humans are responsible for these changes?
• How will temperature, precipitation, severe weather, and other aspects of climate change in my city/state/region over the next several decades?
• Will these changes be steady and gradual, or abrupt?
• Will seasonal and interannual climate variations, like El Niño events, continue the same way or will they be different?
• Why is there so much uncertainty about future changes?
This chapter attempts to answer these questions or explain what additional research would be needed to answer them. The chapters that follow focus on the impacts of climate change on a range of human and environmental systems, the role of these systems in driving climate change, and the state of scientific knowledge regarding actions that could potentially be taken to adapt to or limit the magnitude of climate change in those systems. All of the chapters in Part II follow a similar structure and are more detailed and extensively referenced than the concise overview of climate change science found in Chapter 2. However, these chapters represent only highlights of a broad and extensive collection of scientific research; readers desiring further detail are encouraged to consult other recent assessment reports and the primary literature.
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