Tag: Chinese Academy of Sciences

Heat waves over land worldwide are beginning earlier, ending later, and occurring over longer seasons, while the first heat wave of the season has recently shifted toward faster onset, according to a new study published in Nature Climate Change.

The research was led by scientists from the South China Botanical Garden (SCBG) of the Chinese Academy of Sciences. They analyzed global climate data from 1979 to 2023, focusing on seasonal heatwave onset and end dates, the length of the heatwave season, and the onset speed of the first heatwave each season.

Key findings

On average, the first heatwave of the season began 3.29 days earlier per decade, while the last one ended 5.41 days later per decade. This extended the overall heatwave season by 8.71 days per decade.

Over the full ~45- year period, this translates to the first heatwave advancing by about two weeks, the last occurring roughly 24 days later, and the season lengthening by about 39 days.

Trends were widespread: ~72% of global land areas showed earlier onset, ~79.6% later end dates, and ~92.1% longer seasons. Changes were generally greater in drylands.

From 2001 to 2023, the share of rapid-onset first heatwaves (classified by how quickly temperature peaks within the event) increased significantly in heatwave- affected land areas.

Longer heatwave seasons are also increasing crop exposure to extreme heat during sensitive growth stages (e.g., flowering and silking). From 2001- 2023 versus 1979- 2000, the proportion of crop areas affected rose by 1.6- 13.3% for several major crops, with exposure expanding into earlier and later developmental stages.

Implications

Lead and corresponding author Xu Wenfang noted that prior work has mainly examined whether heatwaves are becoming more frequent, stronger, and longer. This study highlights that timing and development speed are also shifting. Earlier arrival and faster intensification can leave societies and ecosystems with even less time to prepare and adapt.

The authors suggest incorporating heatwave onset/end timing and the speed of the first seasonal event into monitoring, early- warning systems, and risk assessments. This could improve strategies for public health, agriculture, ecosystem management, and energy supply.

This aligns with the phys.org headline snippet (“Heat waves on land start earlier and develop faster, global analysis reveals”) and related recent coverage of expanding heat seasons and stress.

Heatwave early warning systems (HEWS or HHWS, heat- health warning systems) are integrated tools that monitor, forecast, and communicate dangerous heat conditions so governments, health services, communities, and individuals can take timely protective action. They form a core part of broader Heat- Health Action Plans (HHAPs) and multi- hazard early warning frameworks.

Core Components (Four Pillars of Early Warning Systems)

1. Risk knowledge: Understanding who is vulnerable (elderly, children, outdoor workers, people with chronic illnesses, low- income groups), where risks are highest (urban heat islands, drylands), and how heat interacts with factors like humidity, air quality, and socioeconomic conditions.
2. Detection, monitoring, and forecasting: Using meteorological data and models to identify upcoming heat events.
3. Warning dissemination and communication: Clear, actionable alerts delivered through multiple channels (media, apps, SMS, sirens, community networks).
4. Preparedness and response: Pre- planned actions such as opening cooling centres, adjusting work schedules, activating health surveillance, and public messaging.

How They Work

Most systems rely on temperature thresholds (maximum/minimum air temperature sustained for a set number of days), often adjusted for local climate and acclimatization. More advanced ones incorporate:

• Thermal indices (e.g., Heat Index, Wet Bulb Globe Temperature/WBGT, Excess Heat Factor) that account for humidity, wind, and radiation.
• Health-impact thresholds based on mortality/morbidity relationships.
• Graded alerts (e.g., advisory → watch → warning → emergency) with colour codes.
• Lead times typically of 2- 7 days; some systems provide earlier probabilistic outlooks (week 2 or sub-seasonal).

Examples of operational systems:

• United States (NWS HeatRisk): Colour- numeric index based on unusual heat for the location and date; available up to 7 days ahead; expanded nationwide. Complements traditional Excessive Heat Warnings/Advisories.
• Australia: Excess Heat Factor (EHF) metric; aligned with the national Australian Warning System (Advice → Watch and Act → Emergency Warning). Bureau of Meteorology provides multi-day lead times.
• Europe: National systems in France (vigilance levels), Germany (biometeorological), Italy (city-specific impact- based), Spain, and others, often linked to health agencies.
• India: India Meteorological Department (IMD) heatwave forecasts (now 4- 5 days ahead under Mission Mausam); many states have Heat Action Plans with GIS-based mapping and colour- coded alerts.
• South Korea: Newer multi- tier system including emergency alerts at high perceived temperatures.
• Other regions: Expanding in Africa (e.g., Senegal, Burkina Faso), Asia, and Latin America, though coverage remains uneven. preparecenter.org

A 2023 WMO survey found that about 54% of National Meteorological and Hydrological Services issue some form of extreme heat warning. Most are simple temperature-based; fewer use advanced thermal or intensity metrics.

Effectiveness

Evidence shows HEWS can reduce mortality and morbidity when well- designed and linked to action:

• France’s system (post- 2003) is credited with saving thousands of lives in later events.
• Studies report lower ambulance calls and deaths on alert days versus non- alert heat periods.
• Scaling up heat- health warning systems globally could prevent nearly 100,000 deaths annually (WHO/WMO estimates).

Effectiveness depends on accurate forecasts, clear communication, public trust/perception of risk, and the ability of vulnerable groups to act (e.g., access to cooling, ability to stay indoors). Barriers include cost of air conditioning, outdoor work necessities, and heat normalization in some cultures.

Current Limitations and Challenges

Uneven global coverage: Many low- and middle- income countries, especially in Africa and parts of Asia, still lack robust systems.

Metric and threshold issues: Heavy reliance on air temperature alone; thresholds often not regularly updated for a warming climate or local health data.

Communication gaps: Warnings may use technical language, focus on temperature rather than impacts/actions, or fail to reach the most vulnerable.

Action gaps: Warnings are not always linked to pre- agreed, funded response plans; cross- sector coordination (health, energy, labour, agriculture) can be weak.

Forecast skill: Reliable at 1- 7 days in many regions; sub- seasonal (weeks ahead) skill is improving but variable. Onset timing and rapid intensification remain harder to predict precisely.

Limited attention to timing and onset speed: Most systems focus on intensity, duration, and frequency. The September 2026 Nature Climate Change study (“Earlier and faster heatwave onset on land under a warming climate”) shows heatwaves are starting ~3.3 days earlier and ending ~5.4 days later per decade, with seasons lengthening and a recent shift toward faster- onset events (especially in drylands). The authors explicitly call for incorporating timing and onset speed into early- warning systems and risk assessments, as earlier and faster events reduce preparation time.

Recent Developments and Future Directions

WHO and WMO guidance: Original 2015 guidance on Heatwaves and Health is being updated (new version expected around 2026); second edition of Heat- Health Action Plans guidance (2026) emphasizes graded, health- linked thresholds and integration with broader plans.

AI and higher- resolution modelling: Efforts in India (IISc– IMD), China (“MAZU” AI multi- hazard system), and elsewhere to improve localised, impact- based forecasts.

Sub- seasonal to seasonal outlooks: Growing use for earlier preparedness (e.g., resource pre- positioning).

Biodiversity and multi- sector systems: Emerging tools for ecosystem heat warnings; stronger links to agriculture, energy, and labour.

Global initiatives: UN Early Warnings for All (EW4All) aims for universal multi- hazard coverage by 2027; heat is a priority area.

Zurich Climate Resilience Alliance / Red Cross reports (2026) highlight rapid expansion but stress the need for better risk knowledge, refined metrics, inclusive communication, and ensuring warnings trigger real action.

In summary, heatwave early warning systems are expanding and demonstrably save lives, but significant gaps remain in coverage, sophistication (especially health- impact and multi- factor metrics), communication, and response linkage. With heatwaves arriving earlier and intensifying faster, systems that currently emphasise intensity and duration will need to better capture seasonal timing and rapid- onset characteristics to maintain effectiveness. Strengthening the full end- to- end chain, from forecasting to last-mile action, remains the priority.

Earlier and faster heatwave onset on land under a warming climate

Heatwaves threaten ecosystems and societies. While previous studies have focused on frequency, intensity, and duration, long- term changes in timing, onset, ending, and season length, have been poorly understood.

The authors analysed global changes in these metrics and in onset- speed type from 1979 to 2023. Globally:

• Heatwave onset advanced by 3.29 ± 0.12 days per decade.
• Ending was delayed by 5.41 ± 0.12 days per decade.
• The heatwave season lengthened by 8.71 ± 0.17 days per decade.

Trends toward earlier onset, later ending, and longer seasons covered 72.0%, 79.6%, and 92.1% of global land, respectively (statistically significant across 13.4%, 20.3%, and 34.4%). A recent shift toward faster heatwave onset was also observed. Drylands experienced more pronounced timing shifts than humid regions.

These results highlight the need to incorporate heatwave timing into early- warning systems, risk assessments, and proactive adaptation strategies under climate change.

Additional details from the paper

Data sources include ERA5, Berkeley Earth, JRA-3Q, TerraClimate, MODIS land cover, and crop phenology datasets. Processed data and code are available via Figshare.

Figures illustrate heatwave detection and onset- speed classification, temporal/spatial trends in timing metrics, patterns of first- heatwave onset- speed types, and correlations with conventional heatwave metrics (frequency, intensity, duration).

The work builds on prior research showing increases in heatwave frequency, intensity, and duration, while adding the critical dimension of seasonal timing and development speed.

This is the peer- reviewed study underlying the phys.org and press- release coverage from earlier today. The abstract matches the reported numbers exactly. Full- text access is subscription- or institution- based.

Journal information: Nature Climate Change

DOI: 10.1038/s41558-026-02762-2

Provided: Chinese Academy of Sciences

Published: 23 September 2026

Authors: Wenfang Xu,
Donghai Wu,
Philippe Ciais,
Ying-Ping Wang,
Mengtian Huang,
Wenping Yuan&
Juxiu Liu

Abstract

Heatwaves threaten ecosystems and societies. Previous studies have examined their frequency, intensity and duration, whereas long-term changes in heatwave timing—including onset, ending and season length—remain poorly understood, limiting our ability to anticipate heat-related risks. Here we analyse global changes in these metrics and onset-speed type from 1979 to 2023. Globally, heatwave onset advanced by 3.29 ± 0.12 days per decade, ending was delayed by 5.41 ± 0.12 days per decade and the heatwave season lengthened by 8.71 ± 0.17 days per decade. Trends towards earlier onset, later ending and longer heatwave seasons covered 72.0%, 79.6% and 92.1% of global land, respectively, with statistically significant trends across 13.4%, 20.3% and 34.4%. A recent shift towards faster heatwave onset was observed. Drylands experienced more pronounced timing shifts than humid regions. These findings highlight the need to incorporate heatwave timing into early-warning systems, risk assessments and proactive adaptation under climate change.