Ilmuwan Bingung dengan Panas yang Belum Pernah Terjadi Sebelumnya di Tahun 2024

Dalam perkembangan yang luar biasa, tahun 2024 mencatat rekor terpanas pada bulan Juni hingga Agustus, melanjutkan rekor suhu global yang memecahkan rekor yang dimulai pada bulan Juni 2023.

Lonjakan suhu yang tak terduga ini, yang digambarkan oleh para ilmuwan iklim sebagai hal yang merendahkan dan membingungkan, telah mendorong penyelidikan intensif terhadap faktor-faktor penyebabnya.

Tren Suhu Terkini

Dari bulan Juni hingga Agustus 2024, suhu global mencapai rekor tertinggi, melampaui periode yang sama pada tahun 2023. Panas ekstrem ini tidak hanya terjadi pada bulan-bulan musim panas; suhu global mencetak rekor baru selama 15 bulan berturut-turut, dimulai pada Juni 2023 dan berlanjut hingga Agustus 2024, menurut[{” attribute=”” tabindex=”0″ role=”link”>NASA’s Goddard Institute for Space Studies (GISS).

While this prolonged heat wave aligns with the broader warming trend driven by human activities, particularly greenhouse gas emissions, its intensity stunned climate scientists. Gavin Schmidt, the director of GISS, described the unexpected temperature surge in late 2023 as both “humbling” and “confounding” in a commentary published in Nature.

The charts on this page show how much global temperatures in 2023 and 2024 diverged from expectations based on NASA’s temperature record. Roughly a year later, Schmidt and other climatologists are still trying to understand why.

“Warming in 2023 was head-and-shoulders above any other year, and 2024 will be as well,” Schmidt said. “I wish I knew why, but I don’t. We’re still in the process of assessing what happened and if we are seeing a shift in how the climate system operates.”

Predicting Climate Variations

Earth’s air and ocean temperatures during a given year typically reflect a combination of long-term trends, such as those associated with climate change, and shorter-term influences, such as volcanic activity, solar activity, and the state of the ocean.

In late 2022, as he has done each year since 2016, Schmidt used a statistical model to project global temperatures for the coming year. La Niña—which cools sea surface temperatures in the tropical Pacific—was present for the first part of 2023 and should have taken the edge off global temperatures. Schmidt calculated that average 2023 global temperatures would reach about 1.22 degrees Celsius above the baseline, putting it in the top three or four warmest years, but that it would not be a record-breaking year. Scientists at the UK Met Office, Berkeley Earth, and Carbon Brief made similar assessments using a variety of methods.

This chart shows Schmidt’s expectation for how much monthly temperatures from January 2023 to August 2024 would differ from NASA’s 1951-1980 baseline (also known as an anomaly). The expectation (represented as the dashed line in the chart) was based on an equation that calculates global average temperature based on the most recent 20-year rate of warming (about 0.25°C per decade) and NOAA’s sea surface temperature measurements from the tropical Pacific, accounting for a three-month delay for these temperatures to affect the global average. The shaded area shows the range of variability (plus or minus two standard deviations).

“More complex global climate models are helpful to predict long-term warming, but statistical models like these help us project year-to-year variability, which is often dominated by El Niño and La Niña events,” said Zeke Hausfather, a climate scientist at the University of California, Berkeley. Hausfather helps produce the Berkeley Earth global temperature record and also generates annual predictions of global temperature changes based on those data.

Surpassing Expectations

Schmidt’s statistical model—which successfully predicted the global average temperature every year since 2016—underestimated the exceptional heat in 2023, as did the methods used by Hausfather and other climatologists. Schmidt expected global temperature anomalies to peak in February or March 2024 as a lagged response to the additional warming from El Niño. Instead, the anomalous heat emerged well before El Niño had peaked. And the heat came with unexpected intensity—first in the North Atlantic Ocean and then virtually everywhere.

“In September, the record was broken by an absolutely astonishing 0.5 degrees Celsius,” Schmidt said. “That has not happened before in the GISS record.”

The chart above shows how global temperatures calculated from January 2023 to August 2024 differed from NASA’s baseline (1951–1980). The previous record temperature anomalies for each month—set in 2016 and 2020—are indicated by the red dashed line. Starting in June 2023, temperatures exceeded previous records by 0.3 to 0.5°C every month. Although temperature anomalies in 2024 were closer to past anomalies, they continued to break records through August 2024. The global average temperature in September 2024 was 1.26°C above NASA’s baseline—lower than September 2023 but still 0.3°C above any September in the record prior to 2023.

To calculate Earth’s global average temperature changes, NASA scientists analyze data from tens of thousands of meteorological stations on land, plus thousands of instruments on ships and buoys on the ocean surface. The GISS team analyzes this information using methods that account for the varied spacing of temperature stations around the globe and for urban heating effects that could skew the calculations.

Exploring Unforeseen Factors

Since May 2024, Schmidt has been compiling research about possible contributors to the unexpected warmth, including changes in greenhouse gas emissions, incoming radiation from the Sun, airborne particles called aerosols, and cloud cover, as well as the impact of the 2022 Hunga Tonga-Hunga Ha’apai volcanic eruption. However, none of these factors provide what Schmidt and other scientists consider a convincing explanation for the unusual heat in 2023.

Atmospheric greenhouse gas levels have continued to rise, but Schmidt estimates that the extra load since 2022 only accounted for additional warming of about 0.02°C. The Sun was nearing peak activity in 2023, but its roughly 11-year cycle is well measured and not enough to explain the temperature surge either.

Major volcanic eruptions, such as El Chichón in 1982 and Pinatubo in 1991, have caused brief periods of global cooling in the past by lofting aerosols into the stratosphere. And research published in 2024 indicates the eruption in Tonga had a net cooling effect in 2022 and 2023. “If that’s the case, there’s even more warming in the system that needs to be explained,” Schmidt said.

Kemungkinan kontributor lainnya adalah berkurangnya polusi udara. Sebuah tim peneliti yang dipimpin oleh Tianle Yuan, seorang ilmuwan peneliti atmosfer di Pusat Penerbangan Luar Angkasa Goddard NASA, telah menemukan penurunan signifikan dalam polusi aerosol dari pengiriman sejak tahun 2020. Penurunan tersebut bertepatan dengan peraturan internasional baru mengenai kandungan sulfur dalam bahan bakar pengiriman dan dengan penurunan sporadis dalam jumlah. pengiriman karena pandemi virus corona.

Emisi aerosol belerang mendorong pembentukan awan cerah yang memantulkan kembali sinar matahari ke luar angkasa dan memiliki efek pendinginan bersih. Mengurangi polusi mempunyai efek sebaliknya: kecil kemungkinan terbentuknya awan, sehingga dapat menghangatkan iklim. Meskipun para ilmuwan, termasuk Yuan, secara umum sepakat bahwa penurunan emisi belerang kemungkinan besar menyebabkan pemanasan global pada tahun 2023, komunitas ilmiah terus memperdebatkan seberapa besar dampaknya.

“Semua faktor ini mungkin menjelaskan sepersepuluh derajat pemanasan,” kata Schmidt. “Bahkan setelah mempertimbangkan semua penjelasan yang masuk akal, perbedaan antara suhu rata-rata tahunan yang diharapkan dan yang diamati pada tahun 2023 tetap mendekati 0,2°C—kira-kira selisih antara rekor suhu tahunan sebelumnya dan saat ini.”

Menghadapi Realitas Baru

Baik Hausfather maupun Schmidt menyatakan keprihatinannya bahwa perubahan suhu yang tidak terduga ini dapat menandakan perubahan fungsi sistem iklim. Bisa juga karena kombinasi variabilitas iklim dan perubahan sistem, kata Schmidt. “Tidak harus berupa ini-atau.”

Salah satu ketidakpastian terbesar dalam sistem iklim adalah bagaimana aerosol mempengaruhi pembentukan awan, yang pada gilirannya mempengaruhi jumlah radiasi yang dipantulkan kembali ke ruang angkasa. Namun, salah satu tantangan bagi para ilmuwan yang mencoba mengumpulkan apa yang terjadi pada tahun 2023 adalah kurangnya data emisi aerosol global yang diperbarui. “Penilaian emisi aerosol yang dapat diandalkan bergantung pada jaringan yang sebagian besar merupakan upaya yang didorong oleh sukarelawan, dan mungkin diperlukan waktu satu tahun atau lebih sebelum data lengkap dari tahun 2023 tersedia,” kata Schmidt.

Satelit PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) milik NASA, yang diluncurkan pada Februari 2024, dapat membantu menjelaskan ketidakpastian ini. Satelit ini akan membantu para ilmuwan melakukan penilaian global terhadap komposisi berbagai partikel aerosol di atmosfer. Data PACE juga dapat membantu para ilmuwan memahami sifat-sifat awan dan bagaimana aerosol mempengaruhi pembentukan awan, yang penting untuk menciptakan model iklim yang akurat.

Schmidt dan Hausfather mengundang para ilmuwan untuk mendiskusikan penelitian terkait penyebab panas tahun 2023 pada sesi yang mereka selenggarakan pada pertemuan musim gugur American Geophysical Union di Washington, DC, pada tanggal 9–13 Desember 2024.

Peta dan grafik Observatorium Bumi NASA oleh Michala Garrison, berdasarkan data dari Institut Studi Luar Angkasa Goddard NASA. Visualisasi spiral iklim oleh Mark SubbaRao, Pusat Penerbangan Luar Angkasa Goddard NASA/Studio Visualisasi Ilmiah.