The primary purpose of the hockey stick graph in climate science is to visually demonstrate the unprecedented rise in global or Northern Hemisphere temperatures over the past millennium, especially highlighting the sharp increase in temperature during the 20th century that corresponds with industrialization and increased human emissions of greenhouse gases. The graph shows a long period of relatively stable temperatures (the "shaft" of the hockey stick) followed by a rapid upward trend (the "blade"), which serves as compelling evidence that recent warming is exceptional compared to the past 900–1000 years
. This graph was first introduced by Michael Mann and colleagues in the late 1990s and has since become an iconic symbol in the climate change discourse. It has been featured prominently in Intergovernmental Panel on Climate Change (IPCC) reports to support the conclusion that human activity is the dominant cause of recent global warming
. The hockey stick graph synthesizes multiple proxy data sources such as tree rings, ice cores, and sediment records to reconstruct past temperatures quantitatively, providing a long-term context for understanding modern climate change
. In summary, the hockey stick graph's primary purpose is to:
- Illustrate that recent warming is unprecedented in at least the last millennium.
- Provide clear, accessible evidence linking industrial-era human activities to global temperature rise.
- Serve as a key visual tool in climate science communication and policymaking to underscore the urgency of addressing anthropogenic climate change
