Contributed by Mandeep Adhikari:  mandeep@hawaii.edu

Future projections indicated that air temperature would increase 1.3 to 1.8 °C by mid-century and 1.6 to 3.2 °C by the end-century (Zhang et al., 2016; Elison Timm, 2017) at the Dairy Farms (“OK Dairy” and “UP Dairy”) in Hawaii. The agriculture and livestock industries, particularly the dairy subsector in Hawai`i, is vulnerable to climate changes as higher temperatures and less rainfall will have adverse effects on cattle. This article highlights how additional heat stress and forage scarcity due to elevated temperature and reduced rainfall challenge animals’ production and health, forage growth, and ranch management. This work has been published in the journal of Translational Animal Science ( ).

To assess the risk of heat stress on cattle production, monthly Temperature Humidity Index (THIs) were calculated for both locations using the average monthly temperature and humidity data between 1920 and 2019. Results showed that the THI ranged from 64.6 to 70.1 at the “OK Dairy” site, while it ranged from 67.8 to 73.5 at the “UP Dairy” site. The four summer months (June to September) at the “OK Dairy” site were not conducive for high-producing dairy cattle (THI > 68). However, the THIs at the “OK Dairy” site never reached 72 (the critical threshold for low-producing cattle) and mostly remained within the range of 67 to 70, indicating favorable conditions for low-producing dairy cattle throughout the year. The high-producing dairy cows in the “UP Dairy” site were exposed to mild (THI > 68) to moderate (THI > 72) heat stress continuously (14 to 24 h) for several months (April to November). During these periods, THI hardly drops below 68, and therefore the dairy cows in the “UP Dairy” site experience more heat stress in absence of nighttime recovery than in the “OK Dairy” site. Therefore, High milk producing dairy cattle are vulnerable to heat stress at both locations particularly during hottest four months of the calendar year (Jun -Sep).

Rainfall at the “OK Dairy” site is expected to increase over time, while the “UP Dairy” site can be even dryer by the mid-century and the end-century. Empirical results for future forage production indicated that the monthly forage production in the “OK Dairy” site is projected to increase by 6% to 8% by mid-century and 13% to 19% by the end-century. Whereas, the forage production in the “UP Dairy” site is projected to decrease 5% to 8% by mid-century and 10% to 11% by the end-century. These projections revealed that the “UP Dairy” site suffers more from forage scarcity, making ranching activities even more difficult in the future unless irrigation is possible. In contrast, “OK Dairy” sites can be even more productive with abundant grass growth in the future.

This study presents results from an ensemble of regional climate model simulations (periods 1996–2005 and 2026–2035) that are used to examine the effects of both anthropogenic forcing and natural variability associated with the Pacific Decadal Oscillation (PDO) on near-term climate projections for the Hawaiian Islands. The Community Earth System Model Large Ensemble (CESM-LE) is used in conjunction with the Weather Research and Forecasting (WRF) model for downscaling. The climate responses to the PDO and anthropogenic forcing are isolated and analyzed separately. In response to anthropogenic forcing, significant increases in surface air temperature, of ~0.8 K, are projected at low elevations. Stronger warming, of up to 1.3 K, is seen at higher elevation areas. Future climate simulations show significant increases in wet season rainfall, of ~10–20%, along the windward slopes of Big Island and Maui. Rainfall patterns during the positive PDO phase are projected to reverse in sign, leading to drier conditions, by ~10–30%, at many locations. Future climate simulations show daily rainfall extremes will increase, by up to ~10–15%, at many locations. Daily temperature extremes are also projected to increase significantly, by up to 1.4 K. Overall, results indicate that natural variability will continue to contribute to uncertainty in near-term rainfall projections for the Hawaiian Islands, masking the forced signal.

Seasonal WRF output needed to reproduce key results are available at . Daily WRF output is provided at .

Based on a revised and extended multi-station Hawaiʻi Temperature Index (HTI), mean air temperature in the Hawaiian Islands has warmed significantly (p = 0.0007) at 0.05°C/decade over the past 100 years. Significant warming trends for the last century are evident at low (0.056°C/decade, p < 0.001) and high elevations (0.047°C/decade, p < 0.01). A comparison of changes in mean temperature between Hawaiʻi’s Mauna Loa Observatory and Spain’s Mt. Izaña Observatory, both above the local trade wind inversion (TWI) and both experience windward orographic clouds trapped below the upper-level TWI at ~2,000 m, are similar and suggest rapid warming above the TWI for these tropical high mountain islands. Read the by Marie Mckenzie.