The warmest of times, the coldest of times: How climate change is affecting us locally

Introduction

As we look ahead to 2025, ACES is taking this opportunity to reflect on climate trends globally, nationally, and locally in the Roaring Fork Watershed. Moving forward, our newsletter will include more regular updates examining how recent local weather events and climate conditions compare to historical trends. Additionally, as we’re doing here, we’ll continue to place these patterns within the broader context of Earth’s climate system—highlighting when our local experience aligns with global trends and when it diverges.

A Note on datasets

There are many historical climate datasets to choose from, and several factors to consider when selecting one. For global and national analyses, we’re using NOAA temperature anomaly data. The global version of this dataset extends back to 1850, while the national dataset begins in 1895.

For state and local analyses, we’re using a combination of the NOAA Monthly U.S. Climate Gridded Dataset, which begins in 1895, and the gridMet dataset, which begins in 1979. Unlike the other datasets, gridMet data is daily, allowing for a more detailed analysis of extreme events. While all of these datasets incorporate station data (e.g., weather stations at airports or SNOTEL stations), we do not explicitly analyze individual stations. Station data is often messy and requires quality control before it can be used in this type of analysis. For example, if a station is relocated to a different elevation or its equipment is changed, it can introduce a warm or cold bias compared to historical data. The datasets we rely on have already been adjusted to account for these discrepancies.

Roaring Fork Watershed Winter 2025

When viewed as an average, temperatures for the 2024-2025 winter appear relatively unremarkable. While they’re slightly above the 1979-present average, they aren’t remarkable compared to recent winters. But anyone who has spent the winter in the Roaring Fork Watershed knows that averages don’t tell the full story—extremes do—and the data bears this out. If we look instead at one-week average temperatures, 2024-2025 is much more remarkable.

The seven days from January 19 to January 25 were the fourth-coldest winter week on record, with an average temperature of 6℉ (-15℃). What’s even more remarkable is that the last time our watershed experienced a week of such consistent cold was in 1990. If we look at the ten coldest winter weeks, the only one to have occurred in the 21st century was this year. Contrast this with the seven days from February 2 to February 8, which were the warmest winter week on record, with an average temperature of 35℉ (1.85℃). Unfortunately, this isn’t so rare in recent history—all of the ten warmest winter weeks have occurred since 2000. The most recent warm period breaks a record set in December 2021.

While climate change creates significant uncertainty, we can say with high confidence that temperatures have gotten warmer and will continue to do so. It is entirely possible that we will not experience a week as cold as January 19-25 in Aspen again. As record-setting warmth becomes more common, extreme cold is becoming increasingly unlikely.

Looking at precipitation, the story is once again dominated by extreme events. Official winter precipitation totals suggest a much drier season, but this doesn’t fully align with what we’ve experienced on the ground or what we see in current SNOTEL data which shows us at 89% of median snowpack . This discrepancy comes down to two key factors: measurement location and the timing of major storms.

First, SNOTEL stations are located in the upper portions of our watershed, with the lowest station on McClure Pass at 8,763 feet. These stations capture precipitation in the mountains, not across the entire watershed. Second, and more importantly, winter precipitation data excludes the massive snowstorm at the end of November. With 35.77 mm of precipitation, November 26 was the third-highest single-day precipitation total for our watershed. (This metric is biased toward winter storms, as they are more likely to cover large areas of the watershed, unlike summer precipitation, which is typically localized.)

While climate change is predicted to increase the frequency of extreme precipitation events, we are not yet seeing a clear trend of increasing extreme precipitation in our watershed.

Globally, January 2025 was the hottest January on record, surpassing records set in 2023 and 2024. This was a surprise to climate scientists, who had predicted a cooler January due to the shift to La Niña (see below). While this could be random variability, there is concern that scientists have underestimated how much the Earth will warm at current carbon dioxide levels.

Looking Back at 2024

Colorado

Statewide in Colorado, 2024 was the fourth hottest year on record, surpassed (in order) by 2012, 1934, and 2017. Both statewide and locally, 2023 was cooler than the 21st-century average. Despite our above-average temperatures, Colorado has escaped much of the record-setting heat experienced by the majority of Earth’s population over the past two years.

In terms of statewide precipitation, Colorado was slightly above average in 2024.

Roaring Fork Watershed

Temperature

Locally, in the Roaring Fork Watershed, 2024 was the 19th hottest year on record. While this is significantly cooler relative to statewide or global temperatures, it still ranks in the top 25% of warmest years for our watershed. Current temperatures in the Roaring Fork Valley (RFV) have increased approximately 1.5°F relative to the 1971–2000 average.

There is seasonal variation within this warming. Summer and fall have warmed almost 75% more than spring and winter. That was on full display in 2024, when spring was 0.6°F below the historical average spring temperature. If spring were excluded from the 2024 average, it would have been the 12th hottest year on record instead of the 19th.

This aligns with predictions from the latest set of climate models, which suggest that summer and fall in Colorado will warm slightly more than winter and spring.

Looking at temperatures by month, only May and November were cooler than the 1971–2000 average.

Precipitation

Precipitation in 2024 was a mixed bag, which again aligns with climate model predictions for our region. Last year was the 38th wettest year on record, with 31.15 inches of precipitation, compared to the average of 28.59 inches.

Looking at monthly precipitation anomalies, November had the largest anomaly, primarily due to the large snowstorm that occurred during Thanksgiving week. Valleywide averages indicate this was the second-largest single-day precipitation event on record.

Unlike temperature, there are no long-term trends in precipitation amounts in the RFV. The graph below shows precipitation amounts by decade. While precipitation has been decreasing over the past two decades, it remains in line with precipitation amounts from the earlier part of the 20th century.

Once again, this aligns with climate change predictions. Climate models tend to show little to no change in precipitation in Colorado. Moreover, models that do predict changes disagree on the direction (i.e., increasing or decreasing).

It is important to note that precipitation is not the same as moisture availability. As temperatures increase, the atmosphere becomes “thirstier,” causing more moisture to leave ecosystems through both evaporation and transpiration.

Globally and Nationally

Globally, 2024 surpassed 2023 as the hottest year on record. In the contiguous U.S., this was also true. However, when including Alaska and Hawaii, 2024 ranked as the second warmest year on record.

Drivers of High Temperatures

While scientists predicted record or near-record temperatures in 2023 and 2024, both years exceeded expectations—especially 2023. Over the past year, climatologists have been working to identify factors contributing to these discrepancies. While natural variability has played a role, researchers have identified five key contributors to the warming observed over the past two years:

1. Man-made global warming
2. El Niño/La Niña (ENSO)
3. Solar cycles
4. The 2022 Hunga Tonga eruption
5. Marine fuel pollution reduction

The 2024 Global Temperature Report from Berkeley Earth quantifies the relative impact of each of these factors over the past decade.

Among these, man-made global warming and ENSO are the most significant drivers of temperature change. We will examine their impacts in greater detail below.

The solar cycle follows an approximately 11-year pattern of fluctuating solar output and is currently approaching its peak. While this cycle is stronger than the previous peak, it remains in line with 20th-century averages.

The 2022 eruption of the Hunga Tonga volcano may have contributed to recent temperature anomalies, but research suggests both cooling and warming effects, making it difficult to draw firm conclusions about its role.

Finally, in 2020, new international regulations reduced sulfur emissions from large ships by ~85%. Sulfur and other aerosols in the atmosphere reflect incoming solar radiation, leading to a cooling effect. While scientists are confident these changes have influenced global temperatures, further research is needed to fully quantify their impact—hence the multiple estimates shown in the above graph.

El Nino Southern Oscillation

ENSO describes fluctuations in sea surface temperatures in the tropical Pacific Ocean. It cycles between three phases: El Niño (warm phase), neutral, and La Niña (cold phase). Different ENSO phases have regional implications for temperature and precipitation, but on a global scale, El Niño leads to higher temperatures.

El Niño conditions persisted from June 2023 to May 2024, after which ENSO transitioned to neutral conditions. Neutral conditions lasted until November 2024, when La Niña developed, where it remains at the time of this publication.

Globally, record or near-record monthly temperatures persisted through August 2024. After that, temperatures remained well above average but were no longer record-breaking. Looking ahead, scientists expect 2025 to be cooler than 2024 and likely cooler than 2023. Since we are beginning the year in a La Niña phase and without the strong, persistent El Niño of 2023–2024, temperatures are unlikely to surpass those records—though it remains possible.

Man-made Global Warming

Greenhouse gas emissions and associated global warming caused by human activities continue to accelerate, despite efforts to limit emissions.

Complete annual carbon dioxide data for 2024 won’t be available for a few months, but preliminary data suggests that 2024 will be among the top three years for increases in atmospheric CO₂ concentrations. Trends in other greenhouse gases, including methane and nitrous oxide, are similar. As shown in the figure above, these gases have the most significant impact on recent temperature anomalies.

For additional information and more detailed analysis of the global climate in 2024 refer to these resources:

NOAA Global Climate Report
NOAA National Climate Report
European Commission Global Climate Report
Climate Central US and Global Temperature Review
Carbon Brief State of the Climate
Berkeley Earth Global Temperature Report